It's too bad GameStop didn't appear to care more.
For any Stardock created games you have purchased, they would be located on your account page here:
https://store.stardock.com/myaccount/products
For any old Stardock games you may have purchased retail and want to now register, please do so here:
https://www.stardock.com/accounts/register
After, if there are updates for the games, it would be on your account page:
https://store.stardock.com/myaccount/products
For any other games produced by other companies (East India Company for example), and due to Impulse now being owned and operated by Gamestop, you will need to contact [email protected] for access to them.
This link may also be helpful in obtaining serial keys for your purchased games from Impulse: https://www.reddit.com/r/GameSpot/comments/4p1lno/get_a_catalog_refund_or_redeemable_keys_for/