How do i use gimp

I cant open VTF files?

Find a VTF importer/exporter plugin script or do what I do and use VTFEdit instead to open them, then export as .bmp or .tga, edit in GIMP, then convert back to VTF.

I just open VTF’s with Photoshop shrugs

Same here.

Convert the vtf to a pic file that GIMP can open in VTFedit.