I have wondered about this too. I think so--have seen them chase away other species too, but then sometimes they mix and mingle too. I think it's an issue because my yard and gardens are pretty small, and no close neighbors has the nectar or host plants we have here, so it might be an issue of resource protection. I even think I've had a Silver-spotted Skipper try to chase me out of his "property." :)

Hopefully your GTS will come back when the Monarch is either elsewhere or unaware.

This time of the year - August - male Monarchs are definitely territorial and don't like to share. Sort of like hummingbirds. They patrol my yard and don't want to share.

I have seen like up to 8-9 monarchs patroling my yard at a time. Along with polydamas and gulf frititllary. I see one Giant swallowtail that wanted to join the garden but the monarchs would chase them away so I moved the Rue plants outside of the garden hoping theyll lay some eggs. I've had some lucks with the Black Swallowtails laying eggs on them!