Overworld gateway tooltip (the actual gateway tooltip, not gateway floor tooltip) tells you that the gateway is destructable even if you own it or can claim it.
It's a little confusing I agree but unfortunately we can't adjust the tooltip dynamically for this. The line is there to add flavour from a lore perspective.
It's was working properly at some point. In current version of the game any gateway tooltip says it's destructable. I downloaded map "Blaise End" from workshop and it does not have this bug.
Hmmm that's curious then, I wasn't aware that it behaved differently at some point and this honestly shouldn't have been touched by recent updates. I'll take this back for review in 2018.
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