The GS645S hasn't got the sharpest brightest RF patch out there, unfortunately. But it is a nice camera to use.
Some people recommend putting a small square of black material on the main VF window to coincide with the patch, or to make a small dark square using a felt marker. This is supposed to increase contrast a bit and improve matters, but I've never found it worked for me. Some folk swear by it though.
The other option is to cover the whole front of the VF with a slip of contrasting filter gel - blue is good - to increase contrast. This does work for me, but because the GS645S meters through the VF and not through the lens, and I often use the meter, you has to mentally adjust for a "filter factor" ... so I don't do that any more and I live with the slight inadequacy of the RF patch.
Gosh that all sounds a counsel of despair .. but perhaps one of the above will work for you.
And the other thing that might help is changing the VF ring for a different diopter strength - you can use Nikon ones, it's the same thread.