If I banish a unit with gear would the gear pop off?

Yes, when you banish a unit with attached gear, the gear will "pop off" (detach).

According to Rule 435.1 and the consistent rulings across RiftJudge FAQ #2960, #8698, and #8440, when a unit moves from a board zone (like a battlefield) to a non-board zone (like the Banishment zone), any attached gear must detach.

Once the gear detaches, it remains at the last location the unit occupied before it was banished. Because the gear is then an unattached card present at a battlefield, Rule 452.1 and Rule 149.3 dictate that it will be Recalled to your base during the next cleanup.