My rule of thumb regarding these types of questions are: relying on tech to solve such problem usually don’t end well.
As for your problem, maybe just set a reminder on your iPhone that respond to your location and time, so when you leave the gym, reminder will remind you to eat fruit instead of junk food. The catch is the location-based reminder has a minimum range of 100m afaik, and if iPhone cannot determine your location it might not work.
Apple Watch should just carry over whatever you have set up on your iPhone.