B-737 Landing Gear Question

First of all, to avoid the landing config warning, the sequence is gear down, then flap 15.

When to extend flap, drop gear, F15 and finally configure for landing is where real life ops depart from the Boeing manual. Airline pilots will try to avoid level offs, so we may capture the GS from above or below., but either way we are preferably descending, so the trigger of movement of the G/S cue is not really relevant. ATC may ask for 160 kts to 4 or 5 miles, you don't want all that drag hanging out necessarily, perhaps you are flying 3.5º G/S so you want drag earlier. Perhaps you are catching up the aircraft in front and need to maintain separation. If you find yourself very high on the profile, drop the gear, perhaps with no flaps to maintain a steep descent gradient. It really does depend on the situation. On a regular day, I'll probably drop the gear between 2000' and 1500'.

Reference the maneuvers section of the QRH for ILS, VNAV and IAN approaches. The FCOM1 Amplified and Supplementary Procedures cover the detail.