What does "Gaps must have a horizontal span of at least 5.0 cm " mean. Does this mean the track ( vehicle leaves and vehicle land) has to be on the same level? or launching track can be higher and landing track can be lower?

No, the tracks do not have to be on the same level. The "launching" track may be higher than the "landing" track. Measurement is the horizontal change, not the vertical/angular distance, from the end of the "launching" track to the start of the "landing" track.