Windows not airtight?
I feel there is a light draft or visible gaps

That's most likely your problem. If you can feel air blowing around the edges of your windows and/or sliding door, then they're not sealed properly. Same if you can see light through it.
Since you know the window manufacturer and you know they're only 5 years old, you may want to contact them to see if they offer a warranty on the windows/doors. If so, you may get them replaced for free and be done with both the bugs and the heat/AC loss.
If they're no longer under warranty, I'm sure they'd be happy to sell you new ones, but for a far more reasonable price, you can tackle replacing weather stripping.
NOTE: Depending on where the air/light is coming from, you could have issues with the sealing between the window and the exterior of the house. This could be allowing water behind the siding, where it could be pooling and starting to rot your house framing. You may also want to contact the home builder to see if they offer any sort of home warranty and if it's still in effect.