Here are some clues to a mystery number.

The GCF of 18, 36, and me is 2.
I am a multiple of 5.
I am greater than 18 and less than 30.
what is the number?

The mystery number is 20.

Explanation:
The GCF of 18, 36, and 20 is 2.
20 is a multiple of 5.
20 is greater than 18 and less than 30.