You are correct - increasing the GSB value will allow you to use gscope numbers >63. I'm not sure if there is an upper limit.
I know we've definitely supported hundreds of gscope numbers before (I've definitely seen collections with -GSB40 defined). I think that value will support gscopes up to 320.
One thing to note: if you are using Funnelback 15.6 or earlier you need to be careful when changing the GSB setting if you are changing it on a collection that is part of a meta collection.
https://community.funnelback.com/knowledge-base/implementation/Gather-And-Index/meta-collections/meta-collections-incompatible-indexes has more information if this affects you.