required gel for smooth intercoures

I think you are asking what commercial lube is REQUIRED for smooth, problem free sexual intercourse.

The answer can be any of several commercial products.  However, even without commerical products, if the female is properly stimulated or sufficiently aroused, it is possible to have completely satisfactorly intercourse.

If you are looking for a recommended product, one of the stadards is KY Jelly... which is commonly available. However many other lubes are sold in adult bookstores, sex positive shops, or pharmacies or even convenience stores... including "wet", "liquid silk", "astroglide", and "moist."