gearbox teeth erosion

Your picture does not show the tooth on the gear opposite to this tooth. This could just be caused by a defect in the metal of the gear.

Op
Need clearer picture
But appears to be pitting and could be corrosion.
Pitting is contact stress, and lubrication could have failed, or the surface of tooth profiles are not harden correctly.
Contact the gear box manufacture get them involved. The documentation of manufacturing , material and heat treat certifications need evaluation. Must not run this gear box, or catastrophic failure will occur.

Op verify if there was a etch and temper.
If temper etch was not properly neutralized it will corrode.
Or if not tempered correctly will cause hydrogen embrittlement

This may not address your question, but it is a nice reference - see attached.

If only one tooth was affected the first assumption would be that debris passed through it.