You don't have to worry about gap too much on a trainer aircraft. Gap becomes an issue on faster type aircraft where the airflow through the gap creates flutter that can destroy an airframe in seconds. If you have had your aircraft at max speed, then the gap isn't an issue.
However if you're wanting to seal the gap for cosmetic reasons the above explations are on the money.
Be careful you get the edges sealed down well, a gap seal coming loose could cause enough drag on one wing to sprial you in or bind a control surface.
If it aint broke don't fix it
Good Luck