I managed to build pgAdmin against GTK3 on fedora 23. I basically rebuilt fedoras SRPM against wxWidgets 3.0.2, GTK3.
Finally pgAdmin looks good on a HiDPI display! When built against GTK2, pgAdmin is practically useless on HiDPI on linux.
But when i set window scaling to 2 (for HiDPI display) I'm unable to resize any windows and the maximize button is gone. When I change the window scaling back to 1 I can resize again and the maximize button is back but everything looks teeny-tiny