Why would Frota gas hard if she didn't gas against Souza (3rd round was her strongest)? She's also going to be fighting at 125 here, which probably suits her more, instead of 115, so she'll have an easier cut. Frota still should be bigger than Gillian who is really a 115'er but moved up to 125 for TUF.
The last time Gillian fought a girl with better BJJ than her (Mayra Silva) she got subbed in the first round. Gillian beat Macedo but I don't think her Venezuelan black or brown belt compares to girls like Souza and Frota. Frota has fought a few black belts in the past (Maiara and Araujo) and has gotten the better of all the grappling exchanges. Gillian isn't going to win the striking, and if she's successful with her only path to victory (getting it to the ground), she's going to be defending subs and sweeps by Frota. She's really not safe anywhere, and she's not a very good round winner due to her inability or unwillingness to throw strikes on top. Her best chance is to get the back like you mentioned. Entirely possible, but not something you want to bank on when backing a favorite.