AusBonsai

This is a topiary tree in the front yard of my rental property. I think it might be a citrus. The main give away is that I think it has gall wasp. Do these infect any other kind of tree?

Yes it is. You need to cut it off and bin it. and yes it will spread to other citrus trees.
We have Grapefruit, 2 types of mandarins, limes and lemons here in the yard and we are forever cutting it out of the grapefruit and lemons.

No point cutting them off now. They are gone. See all the holes. They have left. Cut any new lumps off as they will have already layed a new lot in your citrus ready to hatch this spring.

Does it only infect citrus? Any other kind of tree? This tree is covered in this, there's almost not a single branch not affected, yet the tree appears very healthy.

I can only find reference to them using citrus and it appears, not all citrus species. Interweb posts tell me this is a native wasp that usually lives on our native fingerlimes in Qld and NSW but with all the yummy citrus trees we have planted has now spread all over the country.