I would very much like to buy the IL game set, but only if it will run on my computer. I have Win7 Ultimate (64-bit) as my OS. I see that Win7 is NOT on the compatible operating systems list on the GOG info page...but there are some threads here that suggest it does work?

Is there anything special (or extra) I need to do/download to make the game run in Win7?

I don't have the GOG version, so I could lose face here :), but I don't see why this wouldn't work on Win7.
Essentailly GOG uses the same corefiles as the retail version updated to version 4.10, the only thing that is different is the installer and that is designed to work on modern OSes.
Best is to not install in %:\Program Files, especially if you later decide to install mods and run the installer as administrator and maybe dissable UAC.

I have win 7 and the GOG version runs just fine for me

Win 7 Pro 64, works without extra tweaking. Of course if you have a modern computer, you would want some mods to make it look nicer. And get more planes. Ultrapack and Dark Blue World are the main ones to get.

I'll go ahead and get the game, install it outside the Program Files area (as an Administrator) and see if it runs OK. If it does, I'll hunt for some mods, if someone can point me in the right direction.

Outstanding, thank you everyone!

And I am pleased to see that IL is ON SALE now! It's a sign from...well, from Gog...but you know what I mean.

Off I go to purchase!

Got the entire IL series for 3.99 USD just now.
Of all the (many) great games I've picked up from GOG, I think this may be the best value ever.
Thanks again everyone for the help!