It is rude yes, but, not resigning in a clearly lost position and playing for another 40 moves wasting your opponents time when you're a piece down and have no counterplayer is even more rude in my opinion, especially in OTB.
Is it rude to say gg before giving mate

Are you always this rude? Or only when you see someone is vulnerable?
Look at your comments for the answer to your question.

Do the rules of chess really require people to stay silent?
Absolutely. And you could read the rules in general, and article 12.6 in particular before asking.
It is forbidden to distract or annoy the opponent in any manner whatsoever.
Of course, this does include talking, with few exclusions (offering a draw, pointing an illegal move and such).
Thank you. I've never read all the rules of chess before. To me it makes more sense to ask someone who knows before reading it all myself. It's sorta like home repairs. I'll ask someone else before I try to do it myself.
That rule seems pretty ridiculous to me. It's forbidden to annoy or distract the opponent, or introduce a source of noise. Which means a mouth breather, or someone who is hungry and has a rumbling stomach would be disqualified. If they have a cough, sneeze, have allergies or even have noisy clothing would mean disqualification. It also means saying good game on the last move before checkmate would not be allowed. No wonder people have a less than favorable view of chess players.

I like to say gg but when I do my opponent wouldn't even see my message because there in another game most likely so I do it before the game ends so they would see it.Is this considered runde or unsportsmanlike?Please tell me
no, there is nothing wrong with saying gg before giving mate.
If you are in a winning position, don't say gg
Are you always this rude? Or only when you see someone is vulnerable?