Well the reason could be bad solder points on the micro sd port, a fragemented micro sd card, a faulty micro sd, broken pins on your 3ds game slot or problems with the processing unit on the gateway card itself.
When you tried another game or micro sd and this isn't the reson, there still remain some potential sources of your problem.
Did you drop your 3ds, the card or used it in an area with a lot of air moisture?
You can also check for problems by disassembling the 3ds or the card but I guess even if this was the reason, you probably couldn't see much.