Mineral spirits or acetone will work.Do a google search and go to the
woodgazette website.They are doing an ongoing test of thinning gorilla
glue and strength of the glue. It appears that either solvent will
work fine,one fellow puts a 1/4' of acetone over the remaining glue in
the bottle before he puts it away.This appears to keep the glue fresh
and not harden in the bottle.I"ll have to try it myself.
mike