Gerrold has more credits on Trek than Norman Spinrad, but given the latter wrote "The Doomsday Machine" I'd say he knows how to write Trek at least as well as Gerrold.
None of Gerrold's or Fontana's shows are in my top twenty favorites, so...
I should've clarified a bit — Gerrold did say he and Fontana were the only ones currently left who know how to write "Star Trek." Because Spinrad isn't around anymo— oh, wait.
Joking aside, "Blood and Fire" (PII version) demonstrated that Gerrold doesn't have the strongest grasp on the material, particularly the character of Kirk.