Gazan Civilians did join the attack, took part in looting and at least in some of the violence. It would be hard to determine the exact number and in some cases to differentiate them from militants.
Before we start with the evidence we need to define what a "Civilian" is. And it's hard to define. Assuming that anyone with a weapon is considered a militant, then in the footage of the attack some terrorist attacked wearing uniform, wearing military gear (usually a vest) and wielding weapons. Some Hamas terrorist attacked wearing civilian clothes with military gear and weapons and some with only civilian clothes and a weapon. At least in one video (not linked here, it's gory) I saw what looked to be a civilian, only later to reveal that he had a pistol holstered. This is also what the speaker in the video defines as a civilian, at around 8:20 in the video she sais:
How could you tell the Hamas from the regular folks from Gaza?
it's very it was very easy by how they how what they were wearing "the soldiers" [...] had an uh like a caramic vest and uh guns and they were dressed in uniforms. And the other civilians were just people like you and me in blue jeans and sneakers some were Barefoot.
Starting from the easiest, see below SFW photos of Gazans in civilian clothes with no visible weapons entering Israel on Oct 7th:
Palestinians break into the Israeli side of Israel-Gaza border fence after gunmen infiltrated areas of southern Israel REUTERS/Mohammed Fayq Abu Mostafa
Palestinians celebrate as an Israeli military vehicle burns after it was hit by Palestinian gunmen who infiltrated areas of southern Israel, at the Israeli side of Israel-Gaza border REUTERS/Yasser Qudih
And a picture of a Palestinian in civilian clothes with no visible military gear holding weapons
A Palestinian gunman holds his weapon after infiltrating the Israeli side of Israel-Gaza border fence REUTERS/Yasser Qudih.
All of the pictures are from a compilation by Reuters with some NSFW photos:
Another picture of the same tank with unarmed Gazans in civilian clothes, armed Gazans in civilian clothes and Armed Gazans with military gear in civilian clothes Abed Rahim Khatib – Anadolu Agency. source: Middle East Monitor
In this SFW video compiling several security camera footage videos we see civilian dressed Gazan looting motorcycles, bikes, some unidentified items from a home a TV and agricultural equipment from Kibutz Be'eri.
And we have this testimony of a soldier that took part in the fighting and pushing back of the terrorists about looters specifically in the grounds of the Nova music festival (where the women in the testimony was when the attack began):
הכוח חוזה באזרחים לא חמושים מעזה שרצים כדי לבזוז, ואוזק אותם. "הם אמרו לנו בעברית שהם באו 'רק' לבזוז, לא לפגוע. כל הכיסים שלהם היו מלאים בטלפונים ובכסף שנלקח מאנשי המסיבה", סיפר, "אבל קולות הירי מסביב המשיכו, אז החלטנו להמשיך הלאה".
The force sees unarmed civilians from Gaza running to loot and handcuffs them. "They said that they 'only' came to loot, not to harm. All of their pockets were filled with phones and money taken from the party goers", he told, "but the sounds of firing around continued so we decided to continue farther".
There are many videos of looting, of Israeli looted cars and equipment in Gaza and of Gazans in Civilian clothes take part in the looting and in the fighting, just one final example of this, is Gazans in civilian clothes take part in the kidnapping of Israelis. In this Insider article there are three (SFW) pictures of three different kidnapping, in two of them we see the kidnapers riding in a looted golf cart like vehicle driven by unarmed men in civilian clothes, and one with an armed gunman in civilian clothes on a motorcycle with the victim.
Another evidence of the looting we have from the fallowing news (source in Hebrew) telling that John Deere remotely disabled 15 Tractors and other agricultural equipment that was looted to the Gaza strip.
ממידע שהגיע לגיקטיים עולה כי בשבת ה-7 באוקטובר נעשה שימוש במערכת גם בישראל, כשהיבואנית של ג'ון דיר בישראל, מפרם סוכנויות, השביתה מרחוק 15 טרקטורים וכלים חקלאיים שונים שהפלסטינים ניסו לבזוז ולהעביר לשטחי הרצועה. בשיחה עם גורם המכיר את הנושא מקרוב, עולה שבשבת בבוקר, החלו להתקבל התראות רבות במערכת על כלים שיצאו מהשטח המוגדר, ועושים את דרכם לרצועת עזה: "יחד עם הגל הראשון נכנסו גם האספסוף. האספסוף, כל דבר שהם יכלו להניע ולקחת הם לקחו… החבר'ה האלה עבדו בקיבוצים, במושבים אז הם מכירים הכל מקרוב…" אומר אותו גורם.
Information that GeekTime obtained shows that on Saturday, the-7th of October the system was used also in Israel, when the importer of John Deere in Israel, Mifram Agencies, remotely deactivated 15 tractors and various agricultural vehicles the Palestinians tried to loot and to transfer to the strip territory. In a conversation with a person familiar with the subject closely reveals that on Saturday morning many indications started to be received about vehicles leaving the designated area and moving towards the Gaza strip: "Together with the first was the rabble entered. The rabble, everything they could start and take they took... there guys worked in the Kibbutzs, the Moshavs so they are closely familiar with everything..." said the familiar person.
Final note on militants in civilian clothes. Unlike well orgenized militaries and even para-militaries where all members wear uniform, the Hamas utilizes militants who wear civilian clothes during military operations, one example can be seen in the fallowing Hamas produced video where a Hamas operative in civilian clothes with no visible weapons or military gear shown firing a mortar.
Lastly we need to put the claim in context. The speaker is a first hand witness and a victim, she is not an official spokesperson nor a historian talking long time after the fact with all the facts known and available. In the video she says that she recognized the terrorists because they wore ceramic (bullet proof) vests. To the best of my knowledge no killed or captured terrorist (and there are over a 1000 of them) wore a bullet proof vest, nor a helmet or any other safety equipment (they also didn't have water or first aid on them). And that the terrorist killed thousands, but we know that the number of killed is around 1700. I wouldn't use this testimony to get exact numbers as she didn't have full knowledge of the general picture, nor didn't try to give it, but told her personal story and her experience. I would be lenient with the exact number of civilian clothed Gazans that entered Israeli territory on Oct. 7th but as there are many photos and videos each with tens of civilians from different places I can say that at least hundreds of Gazans in civilian clothes entered and looted on that day.
All the clickable links in this answer show SFW footage, no bodies or gore in any of them.