i have used GSI acrylic paints for a long time now, and i have used lots of methods of thinning it but i found the 2 best to be: distilled water with some winsor and newton acrylic retarder.OR for an easier way, and most effective for me is just use Tamiya Acrylic thinner. yes,tamiya. even if i mix both GSI and Tamiya acrylics..then mix tamiya thinner it will still work perfectly. GSI acrylics are thicker than Tamiya's, so you will have to use a bit more to thin it. .
P.S.
for proportions try 2:1 or 5:3 for GSI with tamiya acrylic thinner. just look for the "milk" consistency when thinning.