The gather is going to be in the garter itself. But if the lace is gathered it will just be fuller. It all depends on what kind of garter she is looking for. Lace is lace just pick something she would like in looks...it doesn't even have to be lace, it could be organza.
There are probably tons of tutorials out there, here are a few... http://www.do-it-yourself-weddings.c...ng-garter.html
This one has several ideas.... http://fiftieswedding.com/diy-thursd...edding-garter/
and another... http://www.craftstylish.com/item/468...arter/page/all
and keep in mind a bride usually has two garters, one to throw and one as a keepsake