I don't remember but I heard it's like an aggregator of some sort too, right?

GOG the store is just that - a store. They only sell games that have no DRM at all, which means a couple of things. One, they almost never get AAA games at release (the exception being games developed/published by CD Projekt, as CDP owns GOG), and two, there's a high likelihood that GOG will offer game versions that are out of sync with or missing features from the same game sold on other platforms (for example, if a game uses Steamworks for its multiplayer, many devs will just strip out multiplayer altogether for the GOG version rather than patching something new and store-agnostic in).

What you're thinking of with the aggregator is GOG Galaxy, which is their (completely un-required) launcher software. Unlike Steam and EGS, GOG's DRM-free nature means you can just buy games on their site, download the installers directly, and go on about your business. Downloading games, starting games, etc., is all just done manually. If you want a dedicated launcher software similar to the Steam and EGS clients, that's what GOG Galaxy is for. And as a value-add, they implemented aggregator features where you can have it pull in your library from Steam, EGS, EA/Origin, Ubisoft, etc., and just view and launch everything from the one spot. I've generally found Playnite to be a little better at being a one-stop launcher, though everyone's mileage will vary of course.