DXT5 is definitely not used often, so I probably wouldn't try using that unless the GIM was specifically made in the colour format (actually, I never even knew that colour type was used until you showed me).

RGBA8888 = 24-bit+alpha, so if the GimConv script shows that as the colour format, then yes, that's what it is (do be aware that colour palettes for indexed colour will always be in RGBA8888 format).