If that's the recent FF7PC "remake", the rework was apparently done by DotEmu (which is a competitor to GOG) for Square Enix, who released it exclusively on their own site:

http://finalfantasyviipc.com/en

So if you want it, I guess you'll have to buy it from there. It isn't apparently sold in even DotEmu's own site, even though they did the actual work.

Now as other digital stores are also expanding to older classics, it doesn't necessarily make sense for GOG to restrict itself only to that ever-diminishing market.

I don't think DotEmu is behind Final Fantasy's remake. There is no such game in their catalogue.
Besides if DotEmu IS a competitor to GOG that means they are using the same technique as well. (DosBox installers, patches, etc.)
In this case there is a tampering with the game's engine. GOG doesn't do that and if DotEmu makes old games to work by tampering their engines then they cannot be considered as competitors.

Apparently they are. See this earlier post as a reference (in the earlier discussion about the subject in the GOG forum):

http://www.gog.com/forum/general/final_fantasy_vii_pc_released_at_squareenixs_site/post6

and for a more official confirmation in the DotEmu forums:

http://www.dotemu.com/en/forums/pc-games/discussion/final-fantasy-vii-by-dotemu

And as I said, DotEmu didn't release it. They did the "remake" work apparently on request by Square Enix, and Square Enix decided to distribute the game themselves, in order to keep all the money.

Other digital stores, and even the IP rights holders (publishers) themselves, are now also becoming more and more aware that they can make money by selling their older classics. It is not anymore only GOG (and DotEmu) who are on that diminishing market. What's with also with all those HD remakes on other sites etc.

Check the DotEmu site. For their classic PC games, they are very much like GOG, but nowadays probably lagging behind in popularity (but then, they have expanded also to mobile games, while GOG has expanded to newer (mostly indie) PC games).

I think GOG is even selling some games where the compatibility fixes were applied by DotEmu, not GOG (just like e.g. Steam and GamersGate are apparently now selling some old classics where the technical work was done by GOG, ie. they practically are the GOG versions, re-released by the original publishers on those sites, not GOG).

Anyway, this FF7PC remake was apparently a special case, apparently they were working on behalf of Square Enix and got more technical support from them than normally in these GOG/DotEmu cases. But even then, as far as I know the game is not a complete remake. E.g., it still uses the poor old MIDI music from the original PC version etc.

Meh, oldies are released just as often as before it changed to GOG. Just vote on the wishlist what games you want here, even though it don't weight much just like Steam Greenlight, that's still a +1 somewhere and all you can do about it.