There's no truth to the main claims.
The proposed Arms Trade Treaty is not about taking away guns from citizens who own guns
The proposed Arms Trade Treaty is not about changing citizens rights to buy own or sell guns to other citizens.
The proposed Arms Trade Treaty is not about restricting legal trade in arms.
U.N. Arms Trade Treaty Fails On U.S. Opposition After False NRA Gun Rights Threat
Note that the treaty concerns international arms sales, not the rights of any nation's citizens to own arms, nor to sell arms to citizens within it's borders.
The claims:
ignore the legal principle that says no treaty can override the Constitution or U.S. laws.
Forbes reports
a press release issued by the U.N. Office for Disarmament Affairs says that “The outcome will not seek to prohibit citizens of any country from possessing firearms or to interfere with the legal trade in small arms and light weapons.”
Snopes rates a similar text (starting after the Reuters quote) as False, saying it is:
erroneous in all its particulars.
FactCheck looks at the issue as well:
Much of what this e-mail claims is simply false. A "complete ban on all weapons for US citizens" isn’t possible under our Constitution, according to the Supreme Court [...]
Some versions of this claim (including those referenced) include two initial paragraphs attributed from Reuters. These are accurate, from an October 14, 2009 article, but it isn't made clear where the Reuters text stops.