Pokémon: Let's Go, Pikachu! and Pokémon: Let's Go, Eevee! are taking fans on a nostalgia trip through a re-imagined HD Kanto, but despite the familiar setting, there have been plenty of changes. One such change that had many people upset is that gyms now have entrance requirements, such as needing a Water- or Grass-Type Pokémon to enter Brock's gym. Thankfully, new information has just been released that should calm those concerns.
Serebii recently went hands-on with the upcoming games, and they got a chance to take on Brock's gym. Upon doing so, they discovered that the entrance requirement is exactly that and nothing more. As soon as you've shown your Grass- or Water-Type Pokémon to enter the gym, you're free to put them back in your PC box and pull out another monster instead. The Let's Go games allow you to access your PC on the go, so swapping out is no problem.