Everyone deserves a reply, even a non-committal reply 
From the specs your HP has a 4k display. (nice but expensive) Is the external monitor 4k as well ?
This a very-very long shot. If you go to Edit -> Preferences -> Display There is an entry for Monitor Resolution. Gimp will have (probably) detected it automatically for the HP . It might be different for external monitor. You can try a manual entry and a guess or there is a Calibrate function involving measuring a line on the external monitor.
edit A look at the specs, 13.3" diagonal, 4K UHD (3840 x 2160), if the monitor resolution gives a value of say 300 try a manual setting of 150 Restart Gimp. it might work and if not just return the original setting.
Keep visiting, maybe someone has better ideas.
From the specs your HP has a 4k display. (nice but expensive) Is the external monitor 4k as well ?
This a very-very long shot. If you go to Edit -> Preferences -> Display There is an entry for Monitor Resolution. Gimp will have (probably) detected it automatically for the HP . It might be different for external monitor. You can try a manual entry and a guess or there is a Calibrate function involving measuring a line on the external monitor.
edit A look at the specs, 13.3" diagonal, 4K UHD (3840 x 2160), if the monitor resolution gives a value of say 300 try a manual setting of 150 Restart Gimp. it might work and if not just return the original setting.
Keep visiting, maybe someone has better ideas.