We could sort of use a tad more info.But, I'll assume the program is Galaxy.And as I don't use It,sorry can't help you but someone will come along that nose:)

Short answer is, you don't. If you're talking about games you've developed and want to release on GOG.com, there is a submit your game link at the bottom of the page. If you're talking about importing games you own on other platforms, you can't. There is a redeem page for some titles if you have physical copies. But that's only for a select few. That's also in a link (Reclaim your game) at the bottom of the page.