Well, I never got south of Rome in a 720
so I'll skip the Sahara bit.
In an ideal world you would have been descending with both engines in idle since leaving cruise altitude, reducing speed gradually and finally, coming up the glideslope, extending flap 1, then flap 5. At glideslope intercept select gear down and flaps 20 - speed would be max 195 knots before flaps 20 is selected. As speed bleed off (to less than 163 knots) landing flap 30 is selected. So far we've used about 1000' down the glideslope.
On a normal approach it is common SOP (Standard Operating Procedures) to be fully established 1000' above touch-down. That is: gear down, flaps in landing position (30 on a 757), speed on Vref +5 (Vref = 137 KIAS at MLW to 109 at 140.000 lbs) and engines producing positive thrust.
So - gear down comes about 2000'.
Yes, you would have time to retract the gear and try again but just barely - remember this thingy is not stopping by it self so to be safe a go around would most likely be the choice of the day.
Indication problem... hmm, got 2 lamps in that indicator - both failed ? Spare lamps are available.
Overflying the tower for a second opinion ? .... why not .... then why ? They won't be able to tell you if the gear is locked, only what it looks like.
Full emergency procedure both in the cabin and on ground will be required.