I bought these assets a while back with the intention of using them in a character generator in game, however I am unable to format the "body" sprite correctly.

Trawling through the comments people suggest it is due to the double height sprites making the sheet indivisible by 20, and suggest resizing the sheet in GIMP to '1792 x 1280' px.

However even after resizing to this format, I cannot set the Hframes and Vframes or offset to animate properly.
I've included the sprite sheet at it's unedited, original size, as I'm unsure if this resize was correct, along with an outfit sprite sheet (which should supposedly work unedited at Hframe; 56 an Vframe; 21).

If anyone has used these before, or knows where I'm going wrong, help would be greatly appreciated.

I would recommend dividing up the sprites into smaller sheets. Be sure that you have uniform spacing with everything.

Trawling through the comments people suggest it is due to the double height sprites making the sheet indivisible by 20, and suggest resizing the sheet in GIMP to '1792 x 1280' px.

You were in the right path here. If we overlap a grid over the image we see the divisions of the sprites are 32X64:

But the image is 1854x1312 what makes it indivisible by 32x64. What I did is to increase the Canvas (not the image, so the image remains not modified) to make it divisible by 32x64, and selected the corner left-top as the anchor:

Now I can go to Godot and set Hframes: 58, Vframes: 21:

And here you are:

Here you have the Godot test project: