after purchasing new headlights and turn signals and installing them onto my GTS-T my GTR bumper won’t fit correctly on it. Turn signals seem to be pushing it down and leaving a gap underneath headlights and the bumper.

Did I do something wrong? And is the GTR bumper even compatible with the GTST? All help is welcome!

No you didn't do anything wrong. Yes the GTR bumper will fit on the GTS-t you just have to live with the 10mm extra in the wheel well on each side.

If you want it to fit perfectly you will need to procure GTR fenders and GTR turn signals. Otherwise you will have to deal with the overhang.

I have mounted several of these during my time in Japan.