That doesn't mean anything. Does Ging look like a man who would care about the number of stars printed on his card? Would he care to update it or go through whatever examination to have it done? In the first place we don't even know if it directly tells about power. It could refer to the person's position, years in Hunters, or years directly serving the organization like belonging to the Zodiac or working as a trainer/Hunter exam organizer, or just sticking to and developing the Hunter ideology. There also isn't any way to measure a person's level of power since it would vary dozens of percents based on who's the opponent.
As we saw in this last arc, there were truly lousy people working as pro-Hunters when Hisoka and Illumi wiped the floor with scores of them. Even one of the top 10 candidates in the election was one shot killed by Hisoka like a sheep. Gon is worlds above those. For all we know, he might qualify for a single star due to his contribution to the ant incident, but I reckon there are other factors that would prevent it, like being a kid and only having a year of membership under his belt.