With BA costs on a 737 they'll never make money on LGW-GCI. CFE could justify an ATR72 (and did very well) but not an RJ.
Once the ATR's and RJ's are all gone from LGW, the only viable option will be Dash 8's.
If BA do put in 737's (no doubt to naive cheers from the island) it will only be a matter of months before they stage a strategic withdrawal on the basis that they've just realised it's losing money....surprise, surprise!
And GCI really hasn't got the market to support a low-cost carrier with the same kind of frequency as BA (CFE) have been offering, so even if (unlikely) one did step in, it would not satisfy the business community on the island who seem to want high frequency and low fares....totally unsustainable for a market of that size.