Presumably Nature is now only capable of creating bloodless people, those with no life in their veins. And proud of many, lives upon his gains. The subject is still 'Nature', but the meaning of the line is not clear. The word gains is used by Shakespeare elsewhere in the sense of profit, rewards . Here I would interpret it to mean assets, or riches.
Nature has nothing left but the riches of the youth to distribute to the rising generations.
The idea is somewhat far-fetched, and the meaning is in any case dubious. In days long since, before these last so bad. The lament that times are degenerating dates back to at least as far as Hesiod, c.700BC, whose epic poem Works and Days described the gradual decline of the human race from the golden age into the age of Iron. One suspects that here Shakespeare is parodying the tedious complaint of the elderly that, in their day, life, customs, people, behaviour, were all much better. If it is not so, one might wonder if there is perhaps a specific reference here to harsh times, Southampton's imprisonment for example, or plots against the Queen, or famine, or natural disasters.