Your link looks like a set of seals and not the full gasket set.
If you are splitting the cases, you might want to replace the crankshaft bearings. If your seal set does not include a shift shaft seal, you might want to get one of those. The seal might get damaged when you pull the case over the shift shaft.
FWIW, the motor is based on the '88 KX250, and the '88 KX250 head gasket is thinner than the KDX250 gasket. Using the KX head gasket will give more compression and better power, yet it will still run fine on pump premium fuel.