Not much info but I'll tell you what I can remember. Never went down Sima GESM myself but I was caving in the region about a decade ago. From what I can recall at that time the locals were in the process of re-bolting the cave so I assume that it all got done. Think a chap called Marcus Taylor may be the one to talk to as he was filming down there.
At the time GESM was the only real mamma in the area (-1200m or something) and the next deepest caves were in the region of -300m. Most of these ended at sumps and I was sniffing around these with interest as there's obviously a hell of a lot to find. Great area to go and I was always hoping that a club would organise a trip that I happened to tag on to! Exploration to be done but also lots of gay, touristy stuff (Cueva del Gato etc), good climbing and canyoning (I think) and never far from the coast.
Since then I think there's been some pretty major exploration taken place and more deep stuff has been pushed. I was looked after by cavers from Marbella and our main host was a lady called Aracelli (I think). Dodgy memory but I think she used to cave with some of the NPC lot (or maybe Imperial?) so they may still have her contact details. They were extremely helpful and I'd suggest that if you do anything down there (especially in GESM as I think you need to arrange access) then suggest you deal with them.
Marcus Taylor did indeed film down there and produced an excellent DVD, which I think he will still sell you for something like ?12 ... a bargain.
I believe, but may be wrong, that you can't go to the GESM in the Summer season because of bats. I also believe that another cave in the same area has been extended quite a long way in the last 5 years and may now be 500m+ deep.
The final piece of info I know off the top of my head is that the GESM is 1098m deep and has a large (200m??) pitch about half way down.