Note: This isn't amazingly useful, but helps with seeing an overview of all your games in 1 place, and you can compare how much time is spent playing games from each distributor.
Currently it has basic support, but there is an itch.io plugin for the GoG Galaxy desktop app.
https://github.com/Ertego/gog-galaxy-itch.io
The release ZIP is complete but the author has not put everything in the repo, so it does not install properly with the plugin updaters people have made for the Awesome Galaxy collection https://github.com/Mixaill/awesome-gog-galaxy
I used https://github.com/Slashbunny/gog-galaxy-plugin-downloader with a batch file where I added "itch".
This creates the unique folders the updater will use. Then unpack the contents of the ZIP to the correct Galaxy plugin folder.
You don't have to do this but it saves duplication if you then use the updater, because some of the files do actually get pulled but not all.
(C:\Users\USER\AppData\Local\GOG.com\Galaxy\plugins\installed)
It is worth using the batch file with only the plugins you want, because by default that updater will grab all the plugins from the main "awesome" list.
I would rate this as a convenience and no more.
It cannot replace the itch desktop app and requires that you do actually have it installed, though you can launch games with it.
GoG Galaxy is a bloatfest, takes ages to load and depending on the accounts you connect will take ages to finish synchronising.
(Admin I understand you may have mixed feelings about this post due to GoG being a competitor of sorts, however it would seem that the owners of GoG are in the same position, but feel that making connections is worth the risk. Think of it as 2-way publicity or a banner exchange).