I tried Launchbox a short while ago, and it seems that it is not similar with GOG 2 or Playnite. That's more an emulator for different games, but you don't have direct integrations with launchers as Playnite has to import them easy and be able to auto-close the launchers when you quit the games, too.
The main advantages for me right now on Playnite compared to GOG 2 are:
1. Besides Rockstar (that was just added - so it's a bit of a work in progress), the games from other launchers always close the launcher when you close the game. On GOG 2, only Rockstar and Origin always work from my tests, Steam and uPlay sometimes close the launcher on exit, sometimes don't. Epic and Battle.net are never closing when you quit a game from there.
2. The resources used are almost 10 times less than GOG 2:
https://i.imgur.com/bIudqWH.png . Theoretically, after some time the resources used by GOG are decreasing, but it still uses a few times more more resources. And being more simple makes the app a bit faster, too.
3. I like the layout more, the Details View more exactly. It's Steam-like, you have all the games on a left-side list and when you click on them you see details. There are even some themes made by the community, one is similar to the last Steam version.
The main disadvantages:
1. It doesn't have achievements - there is a community plugin for that, but it's kinda beta, there are things not working and it's not directly integrated in it.
2. There is no social aspect on it, you just see your games, details about them, the games are imported automatically (uninstalled and installed ones) and you can play them.
3. You don't have an online account. If you want to save your changes, you'll export your library manually (not a big deal).
On Playnite you can directly install games from uPlay, Steam and Origin. It's not working for Epic, GOG, Battle.net, Rockstar (Rockstar isn't even supported for uninstalled games, you see only the ones installed) - when you click install it just opens that particular launcher. This might change when Playnite 8 will release, but will see.
On GOG you can directly install games from Steam, uPlay, Origin, Battle.net and GOG. It's not working for Rockstar and Epic.
I don't find much of a problem regarding the disadvantages right now. I don't use achievements very much and the social side is not really working on GOG 2 right now, either, so when you click on play you can use the friend list from that particular launcher, directly. GOG has the potential to be better at some point, but it still has a lot of things that are not working properly.