Locate some artist acrylic gel medium in any arts supply of big-box craft materials store. Liquitex, Grumbacher, W&N, etc. all make the stuff.
FWIW, my use of Gator’s Grip glue is that it is exactly the same stuff. In fact, I now just add acrylic gel medium to re-fill my Gator’s Grip glue bottle on my work bench.
To thin either product out, just use a little water to make it the consistency you want.
BTW, the acrylic gel medium can also be used to make water and ice effects in addition to working as an adhesive. For example, you can make really nice icicles by dipping a piece of clear stretched sprue or sewing thread into the stuff repeatedly like dipping a candle in wax.