A Capital Gain is a profit on a Capital Asset. Example: Your house, you paid 100K and sold it for 120K, the 20K is a capital gain.
So if you just made a profit off of a product from your Capital Asset it is just income and you only pay taxes on it if you made enough to have to pay taxes.
Royalty income from oil/gas production is considered to be ordinary income and subject to the taxes just like business profit, W-2 income, etc. Income made from the difference between purchase and sale of a capital investment is a gain on capital investment, thus a “ capital gain” and subject to the tax laws that are applicable.