Send an email to downloadsupport@gamestop.com with your account details asking for a key transfer or refund to your catalog because you've lost access to it.  They should get back to you eventually with steam/origin/whatever keys and an offer to refund any they can't transfer.

I say eventually because when I did it last year it took 5 weeks before they replied to my request. You can try sending it to cs_impulsehelp@gamestop.com instead, might go faster since that was where the actual key email came from. The several years old directions were to send to the other address though so it may not work.

The text I used for my request:

"I have been made aware that I can regain access to my GameStop purchases, either through a key transfer to another distribution service, or a catalog refund, and would like to get this done as I find myself unable to successfully login using the application."

For any non-Stardock games you would need to keep trying GameStop, there is nothing we can do about the games purchased there.

If they are Stardock games that you need help with, please contact our support team.

https://esupport.stardock.com/index.php?/Tickets/Submit

You want to be a little more specific and a little less accusatory?  What game?  Do you have the reg key?  What exactly is your problem?

Hate to be the bearer of bad news, but that is just how companies selling off divisions work. You may not like it, but Stardock has no rights to any of that info any longer. They may be able to help you out with their own games and the games they published. But other companies games sold through Impulse (now Gamestop) are now handled via Gamestop.

You may have the right to be pissed that Stardock sold off Impulse in the first place, but there is nothing they can do with things outside their control at this time. I remember a lot of people worrying about this back when it happened. I believe many started self archiving their game installs so they wouldn't need to rely on Gamestop.

tl;dr Be pissed at Stardock all you want, but you will still need to deal with Gamestop.

Have you checked your Account to see what your purchase history is, so at the least you might figure out what game you're mad about not having access to? 

Have you filed a support ticket?  Historically, Stardock has been good about retrieving inaccessible games.