I pre-ordered Grim Dawn a few years ago and have had the key since. Do I have to buy the game again to use it?
No you should be able to activate the key on steam or GoG.
I tried it on both gog and steam and it came up as invalid
Where did you find your keys? In your email? Check humblebundle if you haven’t already.
I think I got it now. Cheers 
How did you go about figuring this out? I purchased the game years ago and got two keys: an early access key and a final release key. The final release key doesn’t work on GOG or Steam; code is invalid and not even in the proper format…someone told me to check Humble Bundle, but I didn’t purchase it from there, I purchased it from Grim Dawn’s website.
Crate tied their earlier pre-orders with Humble Bundle. I bought the game before kickstarter and I got my key through HB.
That’s weird. I went to the Humble Bundle site and logged in with the same e-mail I used to purchase the game, although I had to make the account, and it doesn’t show Grim Dawn as being one of my purchases…it shows my other purchases, like Indie Bundles and that, but not Grim Dawn. Ughhhh. I contacted their support and Grim Dawn’s. I hope this can get cleared up.
That’s weird. I went to the Humble Bundle site and logged in with the same e-mail I used to purchase the game, although I had to make the account, and it doesn’t show Grim Dawn as being one of my purchases…it shows my other purchases, like Indie Bundles and that, but not Grim Dawn. Ughhhh. I contacted their support and Grim Dawn’s. I hope this can get cleared up.
Or you just use Humble Bundle’s Order Resender, which is probably what Zombok did earlier.
I didn’t purchase through Humble Bundle and I already tried that; only order sent to me was an old indie bundle I had from years ago. I was told via Crate’s support that the old codes weren’t activation codes, but rather order confirmation codes and that Humble Bundle should honor it and give me a Steam code, so I’m waiting on them to get back to me. I have the code; I saved it for years. Hahaha. It should get sorted out. Thanks for the help so far.
you either bought on Steam or (indirectly) through Humble, there was no third option…
If it were Steam, you would have a key, so that leaves Humble.
Go to Humble and get your keys resent, then follow the GD link. On that page you should see a giant ‘Click here to get your Steam keys’ button, the rest should be obvious 
Is it possible that you changed your email since you bought Grim Dawn via the web site?
If not, did you log into your Humble account and look under Keys to see if Grim Dawn shows its key there?
If you haven’t created a Humble account, then do so and then see if the key is there.
Those old codes were only used by Crate as place holders for the eventual keys. Crate sent their data to Humble using the email addresses of the buyers to register with Humble.