Thanks!

SG can't get ownership data from Steam for anything which includes even a single DLC, so GA creators need to add users on Steam to confirm whether they already own the content or not; After friending, this can be done by visiting the store page of the package (assuming one exists, you might be kinda SOL for removed packages) and checking the 'Friends [who] already own this game' page. Anyone who doesn't own the content won't show up on that page, and thus are safe to send to.

Past that, so long as a user hasn't already won the content on SG, SG doesn't in any way block users from entering. Thus, the only limitations on entry are based on the GA creator's desires (as, excepting unusual package circumstances, GA creators have full reroll rights over any package if a winner owns even a single item from the package). SG simply checks against (Not) Received status to determine if the DLC giveaway was properly completed; confirmation of activation is something left to be handled by the GA creator (via adding on Steam, as noted above).

DLCs are considered a bit of a hassle to give away, because of this circumstance, so many of us are all the more grateful for them when they do get posted. On that note- appreciate you putting this up for us! ^.^