1) Pacifica, in her and Grenda's first appearance, teases Grenda about sounding, and I quote, "like a professional wrestler". It is part of her character that she has a masculine-sounding voice.
2) I have gone through every episode that Grenda makes an appearance in and checked the credits (meaning went to a streaming service and skipped to the credits of each episode). The only times Carl Faruolo isn't credited as voicing Grenda, he is listed as an additional voice while Tara Strong is not.
3) Additionally, IMDb and his Wiki page on this Wiki credits Carl Faruolo as voicing Grenda. Tara Strong, on IMDb, is only credited as "additonal voices" on three episodes in season 1, "Double Dipper" being the only one in which Grenda actually makes an appearance. (The other two are "Irrational Treasure" and "Time Traveler's Pig".)