Air carrier Captain reported receiving a low altitude alert from ATC on a night visual approach to DSM.

Narrative

We were on a wide left downwind when cleared for a visual to runway 13. Controller told us to stay within 6 miles. The FAF marker for ILS 13 named CLIVE is 6.8 and has a published altitude of 2600 ft. This was our backup to the visual. The FO aimed on the base leg to fly slightly inside of CLIVE and dialed 2000 for an altitude to descend to. As we were getting close to the localizer (1 mile?) the FO thought he was still too high as his glideslope was showing below us and chose to go down slightly further than planned to join on the localizer. I was busy looking for the traffic we were following when we received a low altitude alert by ATC and simultaneously from the aircraft for landing gear not down (we were at flaps 2). I called to level up and answered ATC's query and noticed my radio altimeter showed approximately 950 ft. The FO climbed approximately 200-300 feet and we finished configuring and running checklist. The rest of the approach and landing was uneventful. Some parameters that contributed to this event was the clearance requiring us to fly inside the marker especially at night. We should have asked for farther out. The FOs dependence on the glideslope while still not in line with the localizer was inaccurate. I should have divided my attention more to inside the cockpit rather than fixating looking for traffic. The FO seemed to be a little fatigued as this was a late night.