Some scientific evidence shows that although dehydration might possibly trigger migraines, it is definitely not a common trigger. Drinking gatorade is supposed to rehydrate you. The scientific evidence is fairly muddy.
This book chapter reviews evidence from many sources. "Taken together, virtually all aspects of life have been suspected to trigger migraine or TTH, but scientific evidence for many of these triggers is poor. . . possibly dehydration may trigger migraine and TTH in some patients." Dehydration is a "possible" trigger for migraines, not a definite one.
When reading scientific research I see different versions of these two statements a lot: Dehydration can trigger migraine in some people, but a huge variety of things can trigger migraine and triggers are usually specific to individuals. If this claim is true, it is probably only true for certain people, possibly only the ones for whom dehydration is a trigger.
It is also worth noting that migraines can trigger increased urination which can lead to dehydration. I personally suffer from migraines, and this is what happens to me. No matter how much water I drink, I piss it out and become dehydrated again. This is only tangentially relevant to the claim. The claim implies that hydration can prevent the migraine before it gets to this stage.
This study, examines water deprivation headaches, but implies they are distinct from migraines. Their research method involved asking people if they got headaches when they were thirsty, which is not the most robust method for a scientific study. To their credit, their discussion and conclusion section did not overstate the strength of their evidence. This paper states that dehydration may contribute to migraine.
This study, surveyed migraine sufferers about common migraine triggers. Their survey did not include dehydration. I believe the researchers made a list of migraine triggers and then asked the survey participants if they experienced the trigger. Although it is possible that some survey participants had dehydration as a trigger and noone asked, it seems likely that the researchers did not believe that dehydration was a common trigger. This seems odd because I found the paper about water deprivation headaches from reading this paper. At best, this is weak evidence against the claim.
This study also surveyed migraine sufferers about common migraine triggers. The researchers chose 15 common triggers for migraines, and sent out a survey asking migraine sufferers whether they experienced those triggers. A key difference in this study, is that they left free space for participants to write in their own triggers. Although dehydration was not on their original list, 10 of the 522 people surveyed, 2%, wrote it in.
This review paper, reviews the research on migraine triggers. Because it focuses on how people cope with their migraines, I don't expect them to go into depth on the less common triggers. They mention in passing that dehydration may lead to headaches, but do not cite their source or go into any detail. They also have a word of caution that I think applies to the study discussed previously,
Individuals can easily be misled with respect to their own triggers. For example, hunger (low blood sugar level) leads to eating certain foods like chocolate, and a headache may ensue. The headache sufferer may interpret such a sequence to mean that chocolate caused the headache, when it was the low blood sugar level that triggered the headache. Once individuals have drawn such conclusions, they often avoid the “trigger” and thus do not have the opportunity to realize that it is not a trigger.
It is possible that the 2% of people who listed dehydration as a trigger, had misidentified their trigger. The same study listed "physical effort (e.g. running)" as a trigger. Running can cause dehydration.
The claim stated that drinking Gatorade before a migraine is established can halt its development. This article discusses how IV hydration with electrolytes should be a primary emergency room treatment for migraines. "Always give fluids first...you want to flood the patients system with 1 to 2 liters of fluid." By the time a patient is in the ER, it is too late to prevent a migraine the way the claim stated, but it may be possible to reverse the migraine with fluids.
This article reviews evidence for a number of ER migraine treatments. In the section titled "Potential Treatments for Acute Migraine in the Emergency Room Setting"
Fluids ... It is our general clinical experience that dehydration worsens migraine or at least makes it harder to treat. ... Unless there is a medical contraindication, liberal fluid replacement is potentially helpful and unlikely to be harmful.
In my opinion, "potentially helpful and unlikely to be harmful" is a great summary of this answer. There is some evidence that implies that the claim is plausible, but I don't see anything that directly supports or contradicts it.