Knocking over the timepiece serves a literal and a figurative purpose.
First, the literal purpose demonstrate just exactly how nervous he is to be with Daisy at this point. To have leaned against the mantle and actually not known exactly what he was leaning on or the effect of where he leaned demonstrates his thoughts are not all clear. This is indicative of nerves in most situations.
Second, it serves the figurative purpose of playing with time. The timepiece is actually knocked off the ledge. Gatsby is trying to turn back time and get to a point with Daisy that has already passed. Hopefully, he will be able to overcome that which has occured since that moment. This is what the rest of the book will set out to prove or disprove.