Manga 444 Bad Company Stacks

This is the order I think works best.

If you just want to read GTO, even without reading previous entries you will understand 98% of the story.

Bad Company happens before Shonan Junai Gumi but I think it's better to read after (also, it released after).

Ino-Head Gargoyle is a spin-off and can be skipped, but it does add something to Saejima as a character.

GT-R is also a spin-off that (in my opinion) doesn't add anything to the story, so it can be skipped.

There is also Shonan Seven, but the only connection it has to GTO franchise is that it takes place in the same region and high school as Shonan Junai Gumi, therefore I didn't add it to the stack.

Order to read