100yds? honestly i wouldn't really use the GSG-5 past 50yds its more of a fun plinker rather than a serious accurate rifle
if your looking for a fun cheap day at the range then i would recommend that you get it
but if your looking into something with more accuracy and versatility i would go with 10/22
the only thing you can really do with your GSG-5 is get a top rail, red dot, retractable stock, and railed handguard.
you might be able to make 5" at 100 yds but you'll need a scope