The screenshots are (k)ubuntu 16.04 Gimp 2.8.22
If you have multiple windows open in various location on the monitor.
1. Edit -> Preferences -> Windows Management then click on Reset Saved Window Positions to Default Values OK that and close Gimp
2. Restart Gimp gives the three separate windows Tools on the left - Image window - Docks on the right
3. Do not move these, location does matter with Gimp.
4. Use Windows -> Single Window Mode to do what it says.
Now size that up to suit your monitor.
If you have multiple windows open in various location on the monitor.
1. Edit -> Preferences -> Windows Management then click on Reset Saved Window Positions to Default Values OK that and close Gimp
2. Restart Gimp gives the three separate windows Tools on the left - Image window - Docks on the right
3. Do not move these, location does matter with Gimp.
4. Use Windows -> Single Window Mode to do what it says.
Now size that up to suit your monitor.