Yes, the OPM manga has come to an end. The creators completed the narrative, providing closure to the fans. Although some might have hoped for more, what was presented was a comprehensive and conclusive tale.
At first, Sans would probably be a bit shocked, but then he'd turn on his laid - back, sarcastic attitude. He might make jokes at Gaster's expense, which could either annoy Gaster or make him respect Sans more. If Gaster tries to harm him, Sans could use his shortcuts to dodge attacks and try to find a way to reverse the situation. He'd also likely be thinking about how this affects his friends in the Underground, especially Papyrus, and try to find a way back to them.
Definitely! The OPM manga offers a lot of action, humor, and interesting storylines. The art style is also top-notch, which adds to the overall enjoyment. But, of course, it depends on your personal taste in manga genres.