I don't believe this has been mentioned yet.
As per the title, the latest update gives you gems to buy one of 4 mega bundles containing 20 heroes. Just update the game, log in, and you will receive the gems. The choice is then yours for which pack you want to pick.
There are tons of threads/articles online about which ones are worth more etc, but I went for the flex bundle to have a few heroes of each type (this is technically the least "valuable" bundle at current character pricing).
Website seems to be down right now, but this should link you to it when the site comes back up.