I guess we have a very different opinion of what Garoppolo might be. I think he's a player that would improve the QB situation the 6 out of 9 teams that lose out on Lance/Fields/Jones (NE, HOU, DEN, NYG, WAS, PHI, CHI, CAR, NO). I don't think he gets traded for less than a 2nd round pick until after the draft.
Given no NE QB, I'd make your suggested trade and feel great about it.
I look at that group and I'm not at all sure I agree, because cost and health matter.
I don't know that he's a better QB straight up than Bridgewater, Fitz, Dalton or Jameis. I think I'd rather have all of those at their cost than Jimmy G on his salary and the picks you give up. HOU isn't paying Jimmy G unless they trade Watson, otherwise the $ makes no sense (even then, how much better is Jimmy than Tyrod?). PHI just traded down from a spot where they were highly likely to get one of the top QBs, if they like Hurts enough to pass on one of these QBs, they like him enough not to trade a premium pick for the right to pay Jimmy G a lot of money.
That leaves NE (I wouldn't trade much for Jimmy, he's better than Cam, but the money matters), DEN (this makes sense, but not at the cost of a good pick, they'd be better off trading up an guaranteeing a rookie), NYG I guess? But is that team really a contender right now? It doesn't feel like it.
That's the problem... sure Jimmy G if he's healthy is an upgrade on the assortment of very cheap QBs these teams have, but that's kind of the point, those teams in part chose not to spend a ton of money or assets on mediocre QBs. Jimmy G is a health risk and mediocre. You can win with a mediocre QB, but it's a lot harder to win with a mediocre QB if you're paying on the higher end of market for him and you give up good picks.
Edit- to be more clear... the problem with trading for Jimmy G is if you're rebuilding, 2/52 for a QB who has 1 full year, and isn't anywhere near elite isn't a good building block, you'd rather look to the draft or wait it out. If you are a win now team, Jimmy is a moderately interesting upgrade if QB is an issue, but his lack of reliability makes you wary (exactly the reason SF is looking to move on) of paying him.