What about games you already own, or have even through other Sites on your own Gaming System?

How does that all work ?

Do you have to repurchase these Games ?

Yes, well pretty much.
There is GOG Connect which allows you to activate certain Steam games on GOG.
Which currently has 0 games.
And Reclaim Your Game which allows you to reclaim games on GOG using the codes from your physical copies. Which hasn't seen an addition in the last decade or so.
Oh and you can activate your copies of Witcher 1 and 2.
Otherwise, repurchasing is the way to go.

What if I already own Tyrian?

Short snarky answer:
If you buy a lawnmower from a store, do you think a competing store is obligated to give you one for free, just because you already paid for a lawnmower once?

Longer, less snarky answer:
GOG actually does have a few systems in place for people to redeem games from elsewhere on GOG. There's the Reclaim Your Game system, which allows you to get a GOG copy of a very short list of games by registering your serial key from your physical copy of the game in question.

Then there is the newly established GOG Connect system, which allows you to connect your Steam account to your GOG account. Periodically, a limited number of games available on both Steam and GOG will be reclaimable on GOG, if you already have the game on Steam. In theory, anyway. After the initial batch of games which were available for 5 days, another 3 came along for a few days, and since then, nothing.

You don't own Tyrian.
Tyrian owns you. Forever.

How many games do you own here?

None, 731 own me.

For me, the answer is always:
"Far too many, yet never enough."

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What's worse? Today I've been spending most of my time playing a game GOG doesn't carry. :P

I know what you mean. Instead of playing any of the games in my account, a whole bunch of which I haven't even downloaded yet, the only game I actually played much recently was AssaultCube, a freeware shooter.

I'm playing Tomb Raider Underworld. GOG really needs what I affectionately call "The TR Trio" here. Anniversary, Legend, and Underworld. These things are so beautiful to look at you're constantly getting killed by baddies because you're too busy admiring the scenery. :P

I'm ranting and derailing. XD OOOPS!

Don't forget this one. (Lots o' the slippery li'l devils, aren't there?)

They still work just like before. The GOG versions don't make your games owned elsewhere not work. No, you don't have to purchase anything. But you can if you want to. Depends what you want.

Well have all the Metro's including the redux's, Just Don't like Steam. Don't like DRM company's. I have thousands of dollars worth of games including a huge long Library of games from over the last 20 or so years, have given away a few and anything older then that except a couple on floppy's i' have given to friends. I've got games that are to old but don't even have codes they didn't have one you just loaded them and played.

And the reason for this topic was to clarify how this works as it just didn't say anywhere that i could find how this worked with games you already owned. And no i don't repurchase games that i own. nor do i have any illegal ones either. Though i do have a lot of games, some are on Steam because i had no choice if i wanted to play them like Fallout NV, Metro 2033, last light and the redux editions.

I'm and Old Gamer i guess been gaming since Arcades were the thing and that's long before home computers. or consoles. And i don't do consoles. only PC gaming and mostly AAA games like fallout Warframe and the like.

So Thank You for the answers ( No Thanks for the snarky ones ) And everyone calls me Dirty Bird, it's even on my Steam account