LGW is single fleet, so doing both long and short haul, but is still Mainline. Single fleet was started in 2006 (October). LGW has a different pay structure than LHR. LGW have basic/hourly pay/night stop payment. LHR have a variety of allowances, which is complicated to explain to outsiders.
At the moment, BA would like to stop transfers from LGW to LHR on current contracts. Watch this space, basically.
For those in the pool, keep swimming, you may well reach your destination later this year.
Gg