Not sure if Gotham is in the same universe, and if it is it's set well before Flash or Arrow.
Sorry to go off-topic, especially nearly 3 weeks after the fact, but Arrow, Flash, Gotham, and Supergirl are all set contemporarily (read: present-day). Gotham just has a timeless Noir aesthetic that the others don't.
We
are dealing with 3 distinct corners of the Multiverse with the four series, though.
Anyway, onto the episode itself. I really enjoyed it, and thought it was the best of the 6 crossover episodes that I've seen, not only because of what it's setting up, but also because of the return of Jay Garrick and Harrsison and Caitlin working together and being all scienc-ey.
I wish we'd seen Laurel or gotten the "she's in Central City " explanation much sooner, but that's my only real nitpick with the episode.
I did think it was kind of funny in an ironic sort of way that sidekicks with loose lips is a thing that's been carried over to both Supergirl AND The Flash, what with James and Cisco both carelessly giving away their superheroic friends' civilian identities and Winn almost following suit, albeit inadvertently.
Again, I really enjoyed the episode and can't wait to see the conclusion of the storyline on tomorrow night's Arrow.