Is Dimir's milling strategy viable in Gatecrash sealed deck? If not, what can they do instead?
Well, it's up to you to figure it out based on your card pool, really.

However, some numbers are handy:


At the start of the game, your opponent will have at least 33 cards in his or her deck (more if there are mulligans).
Each instance of the "mill until you hit a land card" ability will mill something like 2.5 cards on average. That's pretty weak.


What Dimir does have going for it is that a lot of the milling in this set is attached to other stuff -- the common Balustrade Spy is a feeble mill card but also gives you a 2/3 flyer for a reasonable cost, for instance. And you do have access to some repeatable milling, like the prerelease promo Consuming Aberration and lower-rarity carsd like Paranoid Delusions, Sage&#39;s Row Denizen, Undercity Informer, and Duskmantle Guildmage.

I don't think Gatecrash's milling is nearly as explosive as similar decks in Dark Ascension Limited. And Dimir creatures are by far the weakest of all the guilds; if you want to actually play a solid defense, you're likely to need Orzhov or Simic to help you stabilize (Dimir can work well with both those guilds because it offers them the missing component they need to be a control deck -- lots more removal for Simic, countermagic for Orzhov). If I'm mixing guilds like this, though, I'd much rather be trying to win off of cipher+extort than straight-up milling.

Mostly I just don't think the power cards are there in any capacity. It feels like black in AVR -- you can get a reasonable deck going in draft if no one else is in your colors, but the card pool's pretty thin overall. Magic pro Paulo Vitor Damo da Rosa had this to say about Dimir in Sealed, for instance:


  This means Dimir has a bad promo, a bad Keyrune, an OK Guildmage, an
  OK Charm, horrible rares, horrible uncommons, and horrible commons! To
  top it all, pretty much all of the guild pack’s slots are filled with
  cipher cards, which are not very useful—you want mill cards and
  defensive cards, but those won’t be found in that pack. Unless I
  underrate cipher very much, I’d stay away from Dimir, because it
  doesn’t look like you’ll beat anything but the Orzhov decks.
  
  In the end, if you want to play the Dimir colors, I think the best
  alternative is an esper deck. You get the milling from Dimir and the
  defense from Orzhov. If this is your plan, then I think the Orzhov
  pack actually offers better cards for you—you lose the Dimir promo,
  which is better, but instead of useless cipher cards you get 1/4
  defenders and regenerating Thrulls.