Thanks everyone for the contributions. It's interesting but I've heard my tourists say "collect" in this meaning several times. I remember several months ago one of guests asked me when we were at the theatre: "Are we collecting here in the intermission?" and I've replied "Yes, we are". Maybe that guy was from England or Australia, but definitely not from the US, where "collect" has been confirmed to be used very seldom in such contexts.
"Move the tense from "are" to "will be". "collecting" may have the meaning that you give it, but it would be unfamiliar to most Americans (though it is still used in the southern US states to a degree). I suggest that it be written this way -"What's the difference? Both Future Perfect Continuous and Present Continuous are used about future arrangements. If we say "We are gathering here in an hour" and "We will be gathering here in a hour", will the meaning change somehow?