It's easy to add more than one gym to your membership, so you have more flexibility on where and when you use the the gym.
If you choose a Plus membership, you will have access to any gym that is at the same price or lower than your home gym. If you want to add a gym that isn't automatically included, as it is a higher price, just select 'add more gyms' once you've selected the Plus during the join process and choose the additional gym from the 'secondary gym access' list. This will increase your monthly price to the highest priced gym.
Alternatively, if you want access to all gyms, select 'national access'.
Once you're a member, you can alter this list of gyms on the 'Your Gym' section of your Member's Area to fit your needs.