Any Tampermonkey wizards?

Not sure about your codes, but all my scripts are in this format (provided jQuery is present, which SG has already included).

$(document).ready(function() {
    // scripts here
});

Hope this helps.

Wait so Wizards are just male witches?

Why use a script when you can use "steamgifts.com/giveaways/search?level_min=2&level_max=3" for example?

I can't help you with the script, but I wanted to let you know that witches are not female equivalents of wizards. Wizards and witches practice different kinds of magic and for different reasons. I think you can use the word "wizard" for females as well, or you could use "wizardess".

You can get the level with this:

parseInt(document.querySelector('a.nav__button[href*="/account"]').children[1].title).toString()

And you can hide giveaways above your level in the settings.
But if you want to do a script like this you could also add something like this to it:

level = parseInt(document.querySelector('a.nav__button[href*="/account"]').children[1].title).toString();
element = document.links
for (var i=0; i<element.length; i++){
   if(!!element[i].href.match(/\/giveaways\/.+\?/)){
      element[i].href+="&level_max="+level;
   }
}

So the hrefs from an oppened page will already have &level_max (just the ones that are /giveaways/ and have a ? on it, like https://www.steamgifts.com/giveaways/search?page=3).

I tried your script. It is working fine for me.