"Gentle" Gertrude is a part of the zoo attraction at the 3 Stooges Ghost Town.
Gentle Gertrude is a green female alligator, with lime green sclera.
As her name suggests she isn't a danger to anyone; according to Moe, she wouldn't harm a pirahna. She was alarmed as Scooby-Doo was when he dug up from beneath the ground, right in front of her. Gertrude is also feed baby food. Visitors don't bother her either.
She was being let out of her cage at night, along with the other animals. This was presumably Amos Crunch and/or Rhino's doing, so the Three Stooges would be preoccupied with rounding up the animals, while they secretly mined for uranium.
The New Scooby-Doo Movies
It was one of the animals that Shaggy Rogers spotted in the desert, along with a camel and an orangutan. When Scooby saw a mechanical t. rex on a cliff, he quickly dug under the sand, and when his head poked out, he noticed the resting Gertrude next to him, waking her up and frightening them both. The soon Three Stooges arrived and she, as well as the other missing animals, was returned to the amusement park (where formal introductions were made).