what can a gt20 turbo push for boost?

 

Turbos aren't measured in how much they can boost, they are measured in how much air they can flow, in ohter words, how much HP they are good up to...as for the GT20, I don;t know, but I bet someone here does...
If you can find one that is listed here that has similar trim numbers, it should tell you pretty close..
http://www.turbocalculator.com/garrett.html