KTM 2007 250 SX-F coolant leak from cap.?

If it is the head gasket there should be coolant in the crankcase as well, if not it may just be over filled, or the cap is bad. Cooling systems depend on pressure in the system to help raise the boiling point of the coolant,7-15psi in the system is normal depending on the manufacture if the cap isn`t sealing properly it will leak. Another possible cause would be lines installed backwards or a failed thermostat. Google KTM oem parts suppliers KTM parts catalogs are generally diagrams that show placement and some even have the torque specs.