"My question is; Was Paul Castellano going to have him killed after he shot that cop; and after he got indicted in the car-theft ring? We know what happened to Roy DeMeo; Was Gaggi eventually going to end up the same way?? Paul was close Nino so it's possible he gave him a pass?"

I don't think so. Nino was part of Castellano's inner circle, his right hand man seated next to him when he was made boss at Montiglio's house. Remember how the Eppolitos tried to convince Castellano that Nino was dealing drugs, and it got them killed instead. No, Castellano would not allow rules to eliminate his most trusted and closest.