Getting substring of a token in for loop?

Of course that set x=%%g and a substring extraction of x should work, but be aware that if the substring is taken inside a FOR loop, it must be done with ! instead of % (Delayed Expansion):

setlocal EnableDelayedExpansion
for /d %%g in (%windir%\Assembly\gac_msil\*policy*A.D*) do (
set x=%%g
echo !x:~29!
)