The first vine that grows will generally have mostly male flowers, but you can make it produce more female flowers ( thus making more gourds ) by cutting the ends of the vine back.
Left on their own, gourds depend on insects such as bees and beetles for pollination. One problem with this is that gourds cross-pollinate very easily, and the seeds the gourd produces will usually not be true, and will produce a gourd with characteristics of both plants.
Another side effect is that the germination rate of the seeds will be very poor. Normal germination for gourd seeds can be as low as 25%, but by hand pollination, germination can be as high as 80% or more.