I see big red flags here. Yes biting is pretty typical for children when language skills are not fully developed yet, but the severity concerns me. When I worked in a centre, a bite that broke the skin was deemed a "serious occurence" and the Ministry was called in to investigate. Serious stuff.
We used chewy toys/necklaces for biters, and it went everywhere with them, including outdoor time. When I hear that the child is also biting self and gagging himself, that tells me there are probably oral processing issues going on, that are beyond the typical toddler biting phase. This is not a teething issue, due to the severity and length of time the behaviour has persisted. I imagine the other parents are very frustrated as well. Mom and Dad need a chat about the severity of the behaviour and definitely a consult with an Occupational Therapist to assess these issues. I personally would give it a probation timeline for them to seek outside help, and see some improvement, or find care elsewhere. The stress on yourself and the other children must be so draining.