How much mass does the Earth gain from the solar wind?

Not much. The density of the solar wind is only a few dozen atoms per cubic centimeter traveling at about 400 kilometers/sec. This represents a 'flux' of atoms across an earth-sized area of about 5 x 10^26 atoms/sec or for hydrogen atoms, this is about 1 kilogram/sec....70,000 kilograms/day. The problem is that these particles are usually charged, and would be deflected by the earth's magnetic field, so in fact very little of this gets into the atmosphere...perhaps only a few kilograms per day.