It might help if you give information on the old Gimp version (Gimp 2.8.something perhaps)
and your version of linux because Gimp 2.10.21 is now very old latest is Gimp 2.10.38
If you did upgrade from Gimp 2.8 to Gimp 2.10 some of the old setting are migrated and you can reset the interface
Edit -> Preferences -> Interface -> Window Management then click on Reset Saved Window Positions to Default Values
If you used an old version of Gimp in multi-window mode then that is a different story. Give details of the layout.
A couple of very old videos on moving Tabs around but still valid:
https://www.youtube.com/watch?v=PFvV6b2Og7o
and https://www.youtube.com/watch?v=01PeLL1F8Kg
and your version of linux because Gimp 2.10.21 is now very old latest is Gimp 2.10.38
If you did upgrade from Gimp 2.8 to Gimp 2.10 some of the old setting are migrated and you can reset the interface
Edit -> Preferences -> Interface -> Window Management then click on Reset Saved Window Positions to Default Values
If you used an old version of Gimp in multi-window mode then that is a different story. Give details of the layout.
A couple of very old videos on moving Tabs around but still valid:
https://www.youtube.com/watch?v=PFvV6b2Og7o
and https://www.youtube.com/watch?v=01PeLL1F8Kg