A lot has been said already, so just touching on main objective points.
Purely from a roster construction perspective, this is probably the most perfect possible fit outside of getting someone like Ohtani. Varsho is a legit everyday player who is overstretched as a fulltime C, but can easily handle backup duty. This has cascading effects on the roster, as you effectively remove all limitations on Kirk DHing. The team now can and should pinch run for Kirk aggressively in close games. This is normally a trivial factor, but Kirk is so slow that pinch running Merrifield, Biggio, or Kiermaier will add real value over the course of a season. This also makes late inning defensive C replacement feasible during blowouts which we almost never did last year. I don't put a lot of stock in handedness, but he is a LHH that can play every day, which the front office has desperately sought.
People clearly have a problem with Varsho's offensive production, which is fair. He's being compared to Grichuk. His bat could be a liability, and he's borderline unplayable against LHP. With that said, he is a great athlete with that power-speed-defense combo that scouts love. I think he'll age better than someone like Grichuk and has a much better chance of breaking out offensively. Young Jayson Werth coming up as a catcher comes to mind as a best case scenario.
With all that said, Varsho's present value is almost entirely in his defense, which is elite. I'm not sure people appreciate how much better Varsho is over someone like Gurriel. This makes his floor super high even if he doesn't break out offensively. In a fantasy world, you could imagine a scenario for the Jays where basically every position on the field is gold glove calibre except shortstop.
Varsho replaces Gurriel who is a league average player on a cheap contract for 1 year and isn't worth a QO for draft compensation. To me Gurriel is a non-factor in this trade.
Losing Moreno is brutal. His absolute ceiling is a gold glove C with at least an above average MLB bat, so basically a perennial allstar. He's making the league minimum which is going to hurt the team more than you think moving forward. Even if he doesn't reach his potential, it's going to be hard for Moreno to be a total bust.
With the farm system wrecked and payroll maxed out, this is undoubtedly a move to go for a championship within the Guerrero window. This is the type of deal that might look bad in 5 years, but there is huge variance based on how both Varsho and Moreno develop.