i had 3 females drop tails at once (during a hurricane evac.) and it took them about 2 months to fully grow back.
don't know about your gecko but i could not put anything on their tail "nubs" they just freaked out and licked it off or tryed to rub it on the side of the tank irritating the area more.
for my gecko, I put neosporin on hers because it was drying out i did that twie and so far all is good. So if its not drying out then i wouldnt worry about it.