Maverick
Maverick was a mutant that could heal himself. He was enhanced and designed to do it so he could deal with Wolverine!
This was the code that Gabi wanted to tell him. Just like Maverick, Gabi's body has been enhanced too. This was the only reason she would use the word Maverick on the letter!
She was his maverick!
"Clever…. Very clever." Aden smiled. Gabi could use other well-known characters that could heal themselves. However, the name Maverick also had another meaning. It means an independent-minded person. Or a person who refuses to follow the rules of a group that he or she is a member of.
A normal person who would read the letter and search about the word maverick would simply think that it was something that Aden used to call her because she was unorthodox. It was like a pet name.