what is the metal used in microstrip antennas?

Copper is common used.
For PIFA ,it will be electroplated Nickel on the surface for stainless.

you can look the PIFA definition ....
http://www.poynting.co.za/tech_training/ifa.shtml

well, in general, it could be anything, i used aluminium and copper. i even coated them by nicle and cobalt. often they made by copper, becouse the patches should be etched from the board and the boards mettal is often copper.
and, its not depend on the frequency, why you ask just for GSM? it should be just a mettal, better mettals (such as copper) is prefered.
rgds, marti

yes, u can simply use copper or any higher conductivity materials with the least possible loss tangent

hi harmony,u means skin deph loss that degrade the effieciency of antenna as in conductor dielectric loss tanget but loss tangent describe the losses in dielectic not conductor that the espr is complex that have real and imaginary and loss tanget describe bu imaginary over real
regards

hi
you can use copper or alloy of cooper.
however common copper is used.