Running any binary that you can't examine the source of (and confirm it was built from it without modification) is risky. It's mostly a balance of trust and risk. Even developers have been known to insert what we could malware.
That said, if you get your cracked content from a trusted source, I'd say it's generally safe. Otherwise, exercise extreme caution.
Is GMG an official reseller? Maybe I am out of the loop, but I thought they operated in the grey market.
GreenManGaming is a legitimate reseller, in a similar fashion to humble bundle or similar sellers.
I also do not like them because their keys if you are buying on sale for a new launch, their keys are usually UK based as they are UK based and they will not let you play on sooner launch dates, as I learned with Mirrors Edge Catalyst's release when I wanted to play on day 1 and should have bought direct through origin, but instead now I had to wait a week, and they did not offer a refund policy.
So instead I bought the game again on origin and gave away the other key as a free giveaway to spite them 😂