How can I send a gift?

Type gift?
Send an email to the winner.
Register as a friend on Steam and give it to you.
There will be various things.

In any case, I don't know what method you chose.

Type key?
Giveaways - Created
https://www.steamgifts.com/giveaways/created
Game title ”Send” → If there is no problem with the winner→”Sendkey"
The Steam key registered when creating the GA is displayed to the other party. (Or humble gift link)

After that, did the opponent see the key safely? Was it usable?
It ’s a good idea to ask in the comments and see what happens.

If you are sending a key or a URL for a gift link from a site like Humble Bundle, you can send the game directly through steamgifts. You can view the giveaways you created here or on your profile. Click on your giveaway to view the giveaway page and then click "winners" on the left side of the page. If you added the key or gift link when you created the giveaway, it will be show there and you can just click the button that says "send key". If you did not add the key or need to modify it, you can change it from that page and then click send.

If you want to send the key directly to the winner's email address, go to the page I mentioned above to view the winner and it will show you their email address next to their username.

If you are sending a Steam gift directly through Steam, you need to send them a friend request on Steam and then transfer it through Steam. You can click on the winner here to go to their steamgifts profile and then click on the Steam link under their profile picture to get to their Steam profile.

After you send the giveaway, you should mark the giveaway as sent on your created giveaways page. This will notify the winner that they have a game they need to activate.

After you send the game, allow the winner up to 7 days to activate it and mark it as "received".

Hello, greetings on SteamGifts!

You have 3 ways to send a gift.

1. First is send a key. You need to go created giveaways (gibs), view/edit key field and then click "send". You may need to go to giveaway page >> winners (button on left side menu) and then send a key.
2. Second way is it to send a gift via Steam. In this way you need to send friend invite in Steam and wait for the accept. Next, you need to send a gift via Steam.
3. Third way is to send gift via email. You need repeat steps which described in first way and check for email of winner. Last step is to send gift via Steam.

Don't forget to mark key/gift as send.
You also able to check FAQ to get more info.

Have a nice day!