Mariners acquire: Jeff McNeil
Mets acquire: Marco Gonzales, $3M, Kristian Cardozo
Marco is actually pretty underrated on the mound. Think about it this way. Over the five-year stetch from 2018-2022 he only missed 10 starts or so, and put up ERAs of 4.00, 3.99, 3.10, 3.96, and 4.13. Would you pay 2YR/$27.5M for that kind of pitching? He had 3 2.0+ WAR seasons in that time, and if wins go for about $8M, you're still getting a bit of value if Marco gives you around 4 WAR over the next two years.
He's also making 12.5M this season, and then has a $15M club option with no buyout for 2025. He does well, as I mention above, and it's worth keeping him around. If he struggles, then you can get rid of him, the McNeil contract is gone and in Seattle, and you still have one of your minor leaguers at second to take his place.
However, I don't think this can be a straight-up deal. Yes, the Mariners are taking on a lot of money with McNeil (more a lot of risk if he is a .260 hitter), but his floor and ceiling are still higher than Marco's even though they are the same age. I think the Mariners either need to pay a few million per season on Marco, or send over a lower-level prospect. Nothing huge, but one of those guys in the Kuhn/Cardozo/Sanchez level of prospect.
What if the Mets want a better pitcher though?