Medical leave is a leave of absence. If you were still on the seniority list and had full return rights, you were employed, but on extended leave. Personally, I would list the employment on my resume/application without any sort of gap.

If during the interview the question of the gap comes up, simply explain why you weren't flying. An interviewer isn't going to feel deceived. (At least they shouldn't).

Many other folks are faced with similar situations (FMLA, Mil Leave, Medical Leave, etc).