Used them about 10 years ago .... very big fins with heaps of drive and hold. Bit too big for my liking.
They are very slow (quite big), do yourself a favor and get a Quobba set.
Performers are also quite slow, with a mediocre foil profile.
BUT, the Gerry Lopez rears are nice because they are the smallest rears in the FCSII line, and also they have a symmetrical foil, that gives a smoother rail-to-rail to quad setups. So, if you want a tool-less fin set, you may want to pair them with better fronts. I used to have Al Merrick L fronts with Gerry Lopez rears before switching to Quobbas.
For instance I used Al Merrick twins in front with GL rears for micro waves: (i would switch to normal AL fronts in small waves). Al Merrick fins have a more refined foil than the GL, but the AL quad rears were too big for my taste.
Used the Gerry Lopez Quads a few years back. I found them big and slow (as described above) with a little too much grip.
Moved onto Medium FCS II Performers for a while then finally found a set of FCS II Reactors (Medium again...). To begin with I could only find a Thruster set which I used with the Performer Rears but recently found a set of Quad Rears in the Reactor line.
This has been a bit of a game changer... far more reactive. Turns harder and faster with just the right amount of looseness.
I'm 95Kg on a bad day and find the Mediums a good size.