The best way to learn proper English is to read news report, and watch news on TV. Watching TV shows is a great way to learn casual English, slang words, understand culture reference and humor. If you have already watched these shows then you may recall the words used in the following dialogs.

If you were a brain scientist,
you would be busting a GUT right now.
The Big Bang Theory Season 6, Episode 3

I'll only GUT one of them.
Game of Thrones Season 6, Episode 8

There was a time I would have killed all
seven of you just to GUT these three.
Game of Thrones Season 6, Episode 8

Gut him!
Game of Thrones Season 2, Episode 7

What does your GUT tell you?
The Big Bang Theory Season 5, Episode 7