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!
)

On the other hand, if you want to know "How to get the last part (name and extension) of a token in for loop", the answer is: use the ~Name and ~eXtension modifiers in %%g replaceable parameter:

for /d %%g in (%windir%\Assembly\gac_msil\*policy*A.D*) do (
echo %%~NXg
)

A simple

dir /B %windir%\Assembly\gac_msil\*policy*A.D*

should do the trick. If you want to loop over it:

for /f %%g in ('dir /B %windir%\Assembly\gac_msil\*policy*A.D*') do (
    echo %%g
)

I recently had a similar question, and I wanted to compile here the 3 working solutions:

1. Use EnableDelayedExpansion in your batch file, as in the accepted answer

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

2. Use for /f to process the output of a dir command

   for /f "tokens=*" %%g in ('dir /ad /b %windir%\Assembly\gac_msil\*') do @echo %%g
   

3. And finally, the optimal solution in this case, using enhanced variable substitution. See the end of the help at for /? for more information on this syntax.

   for /d %%g in (%windir%\Assembly\gac_msil\*) do @echo %%~nxg