How can I fix mosquito problem in my garage, sealing garage doors
You don't mention what you observed to decide that it's "relatively well sealed".
You can get strips at your local lumber/big box store, which have a built in gasket.  Cut and nail/screw them to the outside frame with gasket slightly compressed against the door.  Do both sides and the top.  That will seal the edges.  If the door bottom doesn't have a gasket, nail a rubber one  on there too.

Example product

That should take care of the door, but my garage is also vented via the soffits, so small flying insects will always find their way in.  If you have them in large numbers, investigate their ingress and any attractants you may have.