Mike Parrish, a businessman and retired Army officer from Chester County, launched his campaign Tuesday, while state Rep. Mark Rozzi of Berks County formed an exploratory committee for a potential bid for the 6th District seat.
Two Republicans, Chester County Commissioner Ryan Costello and Chester County GOP Chairman Val DiGiorgio, have said they're considering running.
Gerlach, a sixth-term lawmaker, announced last week that he plans to step down at the end of his term in next January.
Republican-drawn redistricting changes that took effect after the 2012 election resulted in the 6th District gaining Republican voter registration and losing Democrats. It includes portions of Chester, Montgomery, Berks and Lebanon counties.