There was a Gym Leader either designed before Brock or intended to be before Brock.
In the internal Trainer order, we can note that Bruno precedes Brock. It is possible that at one point in development, this spot belonged to this unreleased Trainer instead.
Cropped image courtesy of Hakuda2:
Note, though it may be a coincidence, he has a similar pose (and position of badge) to Giovanni:
Another character was also revealed, with a hairstyle similar to that of the old man in the final versions (but is numbered on the sketch as 7, where Gym Leader Blaine would be):
There is an article on Hakuda2 about it, in Japanese.
Edit: The first Gym Leader also reminds me Silver, as well as the rival in Let's Go Pikachu and Let's Go Eevee, though it could easily just be a coincidence https://bulbapedia.bulbagarden.net/wiki/File:Lets_Go_Pikachu_Eevee_Rival.png
Edit 2: Additionally, the Sabrina art is not grouped with the Gym Leaders. Notably the index number for Saffron City is 0x0A; the last town programmed into the game after all the others even Pokémon League (0x09), Cinnabar Island (0x08) and Fuchsia City (0x07).
It could be argued Sabrina was not intended to be a Gym Leader, but we have no evidence. Curiously there is also a quiz question in Cinnabar Island that asks "There are 9 certified POKéMON LEAGUE BADGEs?" and there is the old unused key area (because it sets an unused Fly bit) after Saffron City (map 0x0B).