Websites logging in through Steam

Uhm, access to what exactly? As far as I knew all that the sites got through steam login was your user ID and a confirmation that it was you login in. Any extra access, like activating keys for you, is an extra confirmation. So gleam only really has access to knowing you.

If you're not banned from Gleam, then you can remove your Steam account from your Gleam account. If you are banned on Gleam, then you can't remove any accounts on that Gleam account, and adding any of those banned accounts to a new Gleam account will ban any other accounts connected to that Gleam account.

this might be useful atleast to be aware where all is connected.

For Gleam, you can logout via the raffle page. Click the Edit link above the tasks, you can remove Steam account link here.

There is no "special access" to revoke, all a website receives is your steamID (aka 76561197994335129) confirming you are the owner of said account.

The website can then fetch whatever data you have made public on your profile (like nickname, avatar, games, friends, etc.), which is no different from me viewing your public profile page.

This is different from other providers say twitter/twitch/discord/etc., which will hand out info like email, real name, etc. Often you will see a screen from the provider to confirm what info is being shared with the site.