A cheap solenoid will work with garden watering systems and connections. I made an irrigation system that I added to Home Assistant and it’s great.

https://www.bunnings.co.nz/pope-25mm-solenoid-valve-with-flo...

You can get attachments that allow all sorts of jets, drips and spray patterns.

It would probably plug in nicely in place of the water gun. However drilling through the wall and messing with the plumbing isn’t for everyone.