If you buy shares of Permian or any number of royalty trusts you are exchanging your cash for an ownership interest in a producing gas field. How long the field produces will depend on the price of gas and the reserves of the field at that price. Some gas fields have particularly long lives but many don't.
The value of your investment will rise and fall with the price of gas and with expectations over future gas prices. The higher the price of gas, the greater the value of your investment--- as long as it is producing gas. The life of the gas field, however, will eventually end and the investment will ultimately have no value. Oil and Gas investments are often called "cash-flow" investments because your investment has a beginning (when money is invested), a middle (when the investment pays off in annual income), and an end (when no further revenue is produced).
Scott