EVENT_ENTITY is the wrong event as the obstacle is not moving, the player is. EVENT_IMPACT thus is the right choice.
Apart from that on a solid movement code the player('s bounding box) will never touch the floor but hover above it, so your event should not get triggered. What you can do in the gravity code is to use c_ignore and ignore the obstacle's group (assign one) or use IGNORE_FLAG2 and set FLAG2 for those obstacles/ trigger boxes.

Alternatively, a much nicer way probably is to use regions and region_check.