I would like to have the workout option of gardening on my Apple Watch

1. On your Apple Watch, open the Workout app.
2. Tap Other.
3. Complete your workout.
4. When you're done, swipe right, then tap End.
5. Tap Name Workout.
6. Tap the type of workout to gardening, then tap Save.

Next time you start a workout gardening will be in the list.

If you would like to suggest that to Apple, click here....https://www.apple.com/feedback/

I can see no workout called gardening

https://www.macrumors.com/how-to/add-activity-types-apple-watch-workout-app/

You can just use "Other" and skip the naming part.