Can someone explain to a non US resident how Matt Gaetz ousted McCarthy?

I'm pretty sure it's just a simple majority thing, but someone please correct me if I am wrong. 97% of Republican House members != over 50% of the House.

The Republican majority is so thin, that all it took was 8 or 9 of them to vote no when they called the vote (which itself was only callable because it was a shitshow for Republicans to elect a Speaker a year ago and there was a rule change to satisfy the Gaetz crew (i.e. a rule change that permitted them to do exactly this)).

Where are you getting that 97% figure?

Due to a recent rule change that occurred when McCarthy was originally chosen as Speaker, it only takes 1 member to call for a change of speaker. The vote to remove him was 216 to 210, which is a majority.

As part of the negotiation to get Kevin McCarthy to the speakership back in January, Republicans made a rule change in the House which allowed just one member of Congress to bring up for a vote a measure that would let the entire body vote for his ousting.

Pelosi removed this rule when she was speaker, but to be fair she was also really good at getting her party to stay in line.

It will be interesting to see if Republicans take that rule away to avoid another mess if they manage to vote in a new speaker.