Here's the text of the Gnoll's ability (as found on page 317 of the pdf SRD):
Rampage. When the gnoll reduces a creature to 0 hit points with a melee attack on its turn, the gnoll can take a bonus action to move up
to half its speed and make a bite attack.
And Wild Shape says:
You automatically revert if you fall unconscious, drop to 0 hit points, or die.
In order for the druid to revert out of wild shape, they have to drop to 0 hit points. If they do so as a result of the gnoll's attack, it clearly did drop a creature to 0 hit points, and should be able to use Rampage. The fact that the creature suddenly turns into a different one that has more than 0 hit points doesn't matter.