Hi Aviv,
GMAC is gigabit media access control.
The message is referring to AP interface GE1 and not interface radio therefore I believe it is not related with Mesh.
If I understand correctly, it is referring the AP interface GE 1 receiving mismatch frame ethertype/length.
I will suggest first to check cabling, connector, switchport configuration and AP interface GE 1 configuration.
AP techdump file will be very helpful for further analysis.