Great People refers to Great Engineers, Great Scientists, etc. You still need to complete the relevant social policy tree that unlocks the ability to buy a particular Great Person in the Industrial Era and later -- e.g., completing Tradition allows you to buy Great Engineers, Rationalism unlocks Great Scientists, etc.
You can still get a pantheon belief until one other civ has enhanced its religion and the number of pantheons + religions equals the maximum permitted number of religions (e.g., on a Standard sized map only 5 religions can be founded, so if a civ enhances its religion when there is one other founded religion and 3 other pantheons (for a total of five religions + pantheons), no more pantheons can be founded).