Hi Jeremy!
The Garden Grill Restaurant is a great Character Dining Experience in The Land Pavilion within the Future World area at Epcot.
In order to dine at Garden Grill, you will indeed require admittance to Epcot theme park for the day, or have the
Park Hopper Option on your ticket which allows you admittance to multiple theme parks in the same day.
If your plans for the day of your Garden Grill reservation do not currently involve being in Epcot park, you'll want to speak with a Cast Member at Disney Dining and inquire about changing your reservation to best suit your plans. Call 407-WDW-DINE to make changes to your existing reservation if required.
If you'd like to change your Park Tickets to include the Park Hopper Option, and have a Magic Your Way Vacation Package already reserved, call 407-939-7675 to make changes to your park ticket options.
Thanks for visiting the Moms Panel with your question Jeremy.
Wishing you a simply Magical time at Walt Disney World!
Kirsten