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 downloadsupport@gamestop.com 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/