In the defense of Steam, more than 7000 games were released on Steam in 2017. That's what, triple the entire GOG library? Something to the effect of 20 games a day. How can you even hope to advertise each game properly with those kinds of numbers?
You cannot advertise each game, nor should you. However, they could advertise some games which is what curation actually means, i.e. special care for some and let the rest fight for themselves. Or algorithms a la Facebook could just propose you those 300 from the 7000 that might be of special interest to you.
I don't use Steam much, but if they have the money to pay the fixed costs of offering 7000 new games each year where probably 90% of them aren't bought in significant numbers, then it's fully okay with me. It means more choice for me.
How is GoGs relationship with the developers compared to Steam? What kind of cut do the developers get? How does that compare to Steam? How about control of product and what not?
Relationship and cut with publishers/developers will be about the same from all I've heard.
Control of the product is somewhat different. GOG offers DRM free standalone installers, Steam offers not. Pirating GOG games might be a bit easier therefore, if you mean by control of product the ability to prevent illegal access. Keeping on running on your computer under all circumstances might be easier too.
The rest is mostly the same with small differences in policies like the money-back policies or differences in prices and available games.
Any aspect you are especially interested in comparing GOG and Steam?