I am experiencing an issue on the north-west side of my 2-story house: in the morning 2 hours after sunrise we get infested with gnats. There are hundreds of them inside the house at the main level by the sliding doors and on the windows seals. We vacuum them up and it is up again within minutes. After a couple of hours we manage to clear up all of them until the next morning... It is frustrating. Even a window that does not open seem to be impacted. Yesterday I had a glass company send a tech to my house to take a look. He offered to replace the windows but stated he had no clues whether this would help. He said I should talk to a windows company to replace my windows. I noticed that the weather strip on the sliding doors is not in the best shape so I will replace it this weekend. My current solution is to apply painters tape everywhere I feel there is a light draft or visible gaps on the frames. Do I need to replace the windows? The brand is quanex and the glass thermatru. The house is 5 yo and in Southern California.
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.