Since Pokemon: Let's Go, Eevee is, in more ways than one, a revamped remake of Pokemon Yellow, it was expected that a number of core aspects of the original game would continue to be in place. One things most fans expected to remain the same was the original Kanto Gym Leaders, and this appears to be so with the confirmed return of Brock as the Pewter City Gym Leader.
Even though it has yet to be revealed what Pokemon (and at which Levels / Combat Power) Brock will own in Pokemon: Let's Go, Eevee outside of his trusty Onix, many expect him to also have Geodude when in battle. These are the same Pokemon he had in the original First Generation Games (as well as the later remakes of such), and one should expect and prepare to face these same Pokemon. Fortunately, since Brock is the first Gym Leader players will face, his Pokemon shouldn't be all that difficult to defeat with a few Grass Type Pokemon.