They're in my inventory but I can't view them, I pressed exchange items by mistake instead of +Trade and then I declined the gifts and they don't appear, I hope you can solve this problem :)

I currently have 10x Skyrim Legendary and 2x 7 Days to Die, half of those aren't showing (5x skyrim 1x 7 days).

Told you don't decline gifts :(. When you decline the item ID changes and they will be untrackable, typical Steam logic -_-