I have 2 pendient winners for gifts who won't add me on Steam but they're online on SG

As proximus said, by the rules they have 7 days total to claim and redeem the game, most likely they will accept sooner or later. If they have not yet answered after a few days you can post a comment on one of their past sent giveaways for them to receive a notification. If after 7 days they still havent answered/claimed the game you can send a ticket to suport with proof that you tried to contact them (like a screenshot or message in GA as stated above) to have the giveaway rerolled.

What do you have to get nervous about? The worst that can happen is that you get to keep the games, and the giveaways will be rerolled = as if the current winners never existed. Only winners have anything to lose, since they might lose out on the chance to receive the game because of one reason or another. If they avoid you, it's their own fault.