Almost certainly not. California's Proposition 57 was designed to provide parole consideration for nonviolent offenders and to allow CDCR to award additional credits for rehabilitation and programming. The keyword is nonviolent. Prop 57 specifically excludes people with violent convictions or enhancements, and gang enhancements in California are treated as serious sentence aggravators that effectively remove someone from the nonviolent category for purposes of this kind of relief.
A felon in possession of a firearm charge, combined with a gang enhancement and a prior prison term, puts him firmly outside the population Prop 57 was intended to help. The prior prison term enhancement compounds the problem further by signaling to the system that he has already been through incarceration without achieving the behavior change the law is looking for.
The realistic picture here is that he is going to serve most or all of that 85 percent, which on 11 years puts him at roughly 9 years and 4 months. That is the number to plan around. His attorney can review whether any other relief mechanisms apply, such as any changes in how gang enhancements are calculated following more recent California court decisions, but Prop 57 is not the path for his situation.