Looking into buying one of the anniversary packs. However I have one of the games on Steam (but not in gog nor have I linked the libraries) would I still be able to give this one to someone else?

Maybe. It mostly depends if the game is Connect qualified.

"Languages, features and system requirements may vary between games - please check individual product pages for details. If you already own any of the games included in this collection, a redeem code that you can share with a friend will be automatically sent to you via e-mail."

Of course, it doesn't apply if you don't have the game on GOG, unless the game gets added to GOG Connect before the promo ends. If you buy it at that point, you'll get a gift code.

You can try to ask a staff member (through the support page) but with the sale going on it may take a while for them to answer

Tried and got shot down. Oh well. KC:D was worth $25 to me anyways.

1. Buy the Shadow Tactics game from GOG as a normal (i.e., non-gift) purchase. DO NOT START TO DOWNLOAD ANYTHING FROM IT -- NOT EVEN THE EXTRAS. You will not be keeping it.
2. Later, buy the Modern Classics collection in a separate purchase. You should now get a gift code for Blades of the Shogun, since you already own it on GOG (for the moment). You're free to give this code to whomever you wish.
3. Since you haven't yet downloaded any part of your GOG copy of ST:BotS, you should be able to easily get a no-questions-asked refund on it as long as you request it within 30 days of the purchase.