Stop autocorrect from autocorrecting a word even if I re-type it after it has been autocorrected If I type 'grg' it gets autocorrected on the next time I press space to 'gtg'.

If I backspace the 'tg' and then type 'rg' to once again form the word 'grg', it still autocorrects (again) to 'gtg'.

It does that infinitely. Additionally, no suggestions are shown above/under the word as I am typing it so I cannot escape/cancel the autocorrection before it happens.

This question was actually frustrating to write, I found that the only way i could type 'grg' was by typing 'gtg'  first (notice the space), then left-arrowing to the 't', replacing it with an 'r', and then right-arrowing to the end (after the space) and typing the next word.

Can I stop autocorrect from repeatedly correcting the same word even after I've re-typed it?

Looks like this doesn't happen with dictionary words, for example I can type "automatoc", gets auto-corrected to "automatic", and then I can backspace "ic" and type "oc" and it will leave it as-is.

Maybe this is a problem that only happens with learned spellings or so (I think I made macOS "Learn spelling" of gtg)

I don't want to learn the spelling of words like 'grg', because ordinarily these kinds of autocorrections are helpful. However, in the rare situation I am trying to purposefully write such a word, I feel like me re-writing the word should be enough of a hint for the OS to realise what I am after.
Right click to bring this menu and select Learn Spelling. 

Alternatively, you could set up a text replacement in Keyboard Preferences.