I am not cabin crew, but was an FO at EF LGW until a year and a half ago.
Short Haul at LGW used to be called EOG (European Operations at Gatwick), and operated under a different AOC to LHR. As part of a BA efficiency drive, SH LGW was brought under the same AOC as LHR, and consequently, the cabin crew had to become mainline crew. So in name, yes they are mainline. However, this does not mean they can transfer back and forth as they like.
I know people who have worked at SH LGW for the past five years or so. They wish to transfer to LHR and are on the list. However, when (if) they do finally transfer, their seniority will below that of every single new recruit that has gone to LHR since they joined the company! Basically if you transfer from SH LGW to LHR, you keep your date of joining (for staff travel purposes), but you do not retain your seniority for all other purposes.
At the same time as EOG became mainline, my understanding is that it was decided that all future cabin crew recruitment would take place through SH LGW. Then people could transfer to LHR as and when required, and keep their position in seniority because there would be no further new recruits into LHR. However, the freeze on recruitment which was left a bit too long, has meant that training cannot cope with so many courses in and out of LGW, and so recruitment is taking place directly to all areas. And so the old problem of seniority arises...
Long Haul Gatwick is a different kettle of fish. You are properly mainline then, and you retain your seniority when transfering to LHR. The rumours of the merging of short haul and long haul Gatwick are going strong at the moment. In fact I also heard a rumour that LH Gatwick crews are being given just six month initial contracts, so that the ability to transfer to LHR is then BA's decision rather than the employee's. ie at the end of the 6 months the contract can be renewed for Gatwick or Heathrow, as BA sees fit. Not sure I believe this rumour though...
If you want my advice, avoid all the hassle, and go direct to LHR if you get the chance. The waiting list for crews to transfer from SH Gatwick to Heathrow is a long one, and if the merging of SH and LH Gatwick crews occurs, they may never get the chance to transfer to Heathrow.
Finally, SH Gatwick have very few night stops but many night flights. One of my friends who has been there for 5 years, gets one or two night stops a month (and that is bidding for them, and she is pretty senior). Fewer night stops equates to less allowances... LHR allowances are apparently much better (although I know very little about their structure).
Hope this helps.