My (limited) experience is that a GTAW arc is too hot to braze with very successfully. It tends to either melt the base metal, or vapourize the filler.

Where I have used it successfully was on Cu (big heat sink) where the GTAW arc was used to heat the Cu base metal locally while another person was melting the braze (silver solder in my case) with an Oxy-Acetylene flame.

Maybe someone else has successfully brazed steel with GTAW?

Hope this helps

Regards
Niekie Jooste

Google search for galvanized joining turned this up.
http://www.wisconsinwireworks.com/gmaw_weld.htm
Sounds like what you are looking for. (I don't work for them.)

The time proven way of joining galvanized is solder.

Bill