I use a Loca8tor, but it's not a GPS. It beeps if cat is within 200m - unless there are thick walls or cellars involved. It helped me find MCat in a gale, when he was terrified and hiding. The receiver is the size of a cc, as long as you don't mind the neighbours thinking you're unhinged, spinning in slow circles, trying to pick up a signal.
When mine gives out, I was thinking a Tractive. They're quite bulky, but they seem to be inventing smaller ones.
I always put them on a quick release snap off collar, mind.