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.
The UN Arms Trade Treaty, Article 6, prohibits sales to
- Those subject to UN Embargo (e.g. N.Korea?)
- Illicit arms dealers (e.g. suppliers to Al-Qaeda?)
- People known to be planning genocide (e.g. Akuzu in Rwanda?)
Article 7 requires the exporting country to assess if the recipient will use the arms to undermine peace or violate international law.
Here's the relevant text from http://www.un.org/disarmament/ATT/docs/ATT_text_(As_adopted_by_the_GA)-E.pdf
Article 6
Prohibitions
A State Party shall not authorize any transfer of conventional arms covered
under Article 2 (1) or of items covered under Article 3 or Article 4, if the transfer
would violate its obligations under measures adopted by the United Nations Security
Council acting under Chapter VII of the Charter of the United Nations, in particular
arms embargoes.
A State Party shall not authorize any transfer of conventional arms covered
under Article 2 (1) or of items covered under Article 3 or Article 4, if the transfer
would violate its relevant international obligations under international agreements to
which it is a Party, in particular those relating to the transfer of, or illicit trafficking
in, conventional arms.
A State Party shall not authorize any transfer of conventional arms covered
under Article 2 (1) or of items covered under Article 3 or Article 4, if it has
knowledge at the time of authorization that the arms or items would be used in the
commission of genocide, crimes against humanity, grave breaches of the Geneva
Conventions of 1949, attacks directed against civilian objects or civilians protected
as such, or other war crimes as defined by international agreements to which it is a
Party.