The minus gages are the most commonly used. They are .0002 under the nominal size printed on them. If they were on size, they would be a press fit in the hole, and would not fit.
The plus sizes are .0002 over. If you have both plus and minus, you can effectively measure to .0005.
Once you use one or the other for a while, you can estimate pretty close to .0005 anyway. If the minus pin just barely goes in, that's the hole size. If it's a bit wobbly in the hole, but the next thou. bigger won't go, it's somewhere between the 2 sizes.
I bought a set from .250 to .500 off Ebay several years ago for a great price because they were all jumbled up. An hour ffixing up the deal that held them in order, then some time with my optivisor sorting them out and I am good to go. I have also picked up a set from .061 to .025.