I use gtag manager to push unique user_id from my site to ga. I wanna get it back like client_id when a user browse my site.
I definitely know that gtag does not allow to do that. also there is a dataLayer which can be queried like that
google_tag_manager['<container-id>'].dataLayer.get('gtm.start') but I've not found a solution to get user_id back.
Is it possible somehow to get back this user_id or it works only in one way to enrich ga?
In the real time, it works one way only. It's your responsibility to save whatever you need appropriately.
If it's shifted in time sufficiently (about 2 days shift), you theoretically could use the Data API or a BQ export to pull it by something like GA client id. But that's way too cumbersome. Much easier to preserve the user id in the normal way.