iMac Summer 2001 G3 CRT Issue
I doubt it's the gun that's the issue, but without getting actual hands-on, it's impossible to tell for sure.

It could be something as simple as color being out of sync and a simple degauss should correct.  Pass a magnet across the front of the screen; that should clear things up.  You don't need anything powerful.  I've used re-purposed speaker magnets back in the day.  I have also seen people use tool magnetizers with the added benefit that if it doesn't work, you still have a functional tool for future magnetizing needs (from $2 to $10 USD).

If the degauss fails, it may be a component or a faulty solder joint that controls the color hue/saturation.  At this point, it would be best to take it to a pro who specializes in this type of diagnostic/repair.    It's not a difficult repair and shouldn't be costly at all.  It's just that you will be working with extremely high voltages and can be quite dangerous for the novice.