You might think that manufacturers would make bags that would be more universally applicable, but it doesn't seem to be the case (no pun intended). I suppose every bike has slightly differing space available to make the bags bigger and more tidy looking without making them gosh-awful big.
BMW used to make bags that would fit most anything they made with a few exceptions for models like the R80G/S and R80ST (which used the same bags but different from everything else). It's a shame, but they seem to have gotten in line with everyone else and make bags that fit only 1 model. rather than a universal bag with different mounting hardware.
I suppose you've checked out Givi. I don't know if they do brackets for an F800GT. When all else fails, you could sell the VFR to buy the BMW bags.
Yes Geoffrey I read on another BMW forum (didn't bookmark it darn it) that several other models used the same bags. I just have to figure it out again. I haven't looked at GIVI really, main thing being the bike came with the BMW mounts. I'm no wrencher, so don't know about taking them off, installing others and then trying to sell. Did see some soft ones from SW-Mototech (or whatever there name is) that looked cool, and about half the price.
Now, as for selling the VFR - why I outta wash your mouth out with soap. Don't let Little Red hear you talking like that. But seriously, that bike has been so good to me, and still has less than 25K on the odometer (and it's going on 16 years old), I just couldn't sell it. Love the sound of the engine and pipe way too much.