The lens on the Fujica GS645 is not threaded for filters, nor can it mount slip-on filters. There is, however, a groove on the front of the lens housing that can be used to mount things. This filter holder engages that groove.

This filter adapter is threaded for M40.5x0.5 filters (because I have lots of them for other cameras). 

Two examples here, one printed in red PETG that looks weird and one in a much more serious looking black CF reinforced PETG. The devices are a bit difficult to get mounted and dismounted, so don't beat your camera up. Folding optics are not very strong.