Noob Giveaway Questions

No, only Steam keys, you have to find another way to give that Origin key away, either as a forum drop here or through another site.
For the latter it's almost the same, though you can risk giving it away and in the worst case deal with a 'not received' or, as said before, give it away in the forums to be safe.

No you can't because the winner should activate the key on their steam account, if they don't they will be banned

and if you have an old key you are not sure of, you can make a thread about it, making it invite-only, and ask people to enter only if they agree with you to delete the giveaway if the key was not good

Your answer is in the guidelines section of the FAQ if you want to see it yourself.

. All giveaways should be for Steam redeemable gifts or Steam redeemable keys

Only Steam keys\gifts are allowed to be given away on Steamgifts.

All giveaways should be for Steam redeemable gifts or Steam redeemable keys.
Beta keys, guest passes, coupons, and free games should not be given away.

These non Steam keys can of course be key dropped in the SG discussions or given by Steam messaging to one of your buddies.

For example: here

As noted, you can drop old keys in the "give directly to bots" thread linked by LostSoul above, you can make a thread and pick someone to gift the game to directly, or you can create a group or invite giveaway and request people abide by your request for them to agree to a deletion if the key is no longer valid. An alternative is to find a reputable trader (generally, 50+ positive feedback on SteamTrades or Barter.vg) who wants the game in question, and then offer to trade your game to them first. If they can activate your game, then they can trade you a game of equal contribution value for you to put up here- a key which, due to the trader not willing to end up with negative trade feedback, you can have more confidence in working.

As far as your first question, non-Steam keys cannot be verified by the site, nor equated properly against Steam games, meaning the site cannot allow for any non-Steam-redeemable giveaways. Again, trading your keys to traders for Steam keys would be your best bet there- just keep in mind that (except when the game is exclusive to the platform in question) Origin keys are valued lower than Steam keys, and Uplay keys are valued at nearly nothing. Unless you're trading for junky but high CV games, you're likely going to have to take a disadvantageous trade [in terms of relative retail value and bundling status]. GOG keys are more erratic- Steam diehards value them as low as Origin keys, while those more inclined to GOG may actually value them higher than Steam keys.

Okay thanks for the advice. Better safe than sorry with those keys, so I will be giving them away or trading them if I find someone who wants them.