"When should you put the gear down on a retractable gear airplane?" - Bob W.
“I prefer and have always taught students to extend the landing gear before downwind entry. This slows the aircraft and allows trimming for the gear down condition, as well as allowing a slower speed for purposes of blending with other traffic. I say this assuming a VFR flight.
On an IFR flight, assuming an instrument approach is being performed, it is best to extend the gear at glideslope intercept for a precision approach or at the final approach fix on a non-precision approach. Doing so will help you set up a stabilized approach.”