Nope, the web interface can't list games from other platforms, and I need to tag even those.
The game aggregation that Galaxy does is not present in every other clients I ever seen (not even on Playnite), if I have the same game on GOG and STEAM, and I tag the GOG version as "COMPLETED", it will be seen as completed even the STEAM version. It's super useful to filter already played games or tag games only once for all the platforms.