I would extrapolate from existing GPCR-targeting drugs, but the list is so long I can't really give a survey. As a protein family, GPCRs do a lot of different things but basically it boils down to signal transduction: there's something outside a cell and the cell wants to know that so it can respond. GPCRs mediate the binding of the thing outside to the surface of the cell, and transduct the information into the cell, delivering a signal to the nucleus, where the transcriptional machinery is manipulated to make new proteins (or stop making proteins). I left out some steps. This allows the cell to be responsive to the outside in a secure way, without the outside molecule coming into the cell.

The 3d structure of GPCRs is intimately associated with the cell membrane, with one end sticking out into the extracellular environment, and another sticking out into the interior of the cell. Simply getting that sort of protein, which is not soluble in water, was a massive challenge and the first structures didn't arrive until the early 2000s, when people learned how to crystallize GPCRs using detergents and other solvents that simulated cell membranes.

So many diseases are caused by misregulation of signal trandsduction... let's take some examples. Vasopressin is a drug that also happens to be a natural body product. In some diseases, people don't make enough vasopressin to regular the kidneys properly, leading to a form of diabetes, and simply giving people more of it helps reduce the symptoms. I guess in this case (not certain), people have mutations in the GPCR that receives the vasopressin signal that attenuates the signal "too much" and by just dosing the patient with a lot more, more signal gets sent inside the cell.

I think at least 10 nobel prizes in medicine have been awarded for research related to GPCRs, which has uncovered a wide range of medically relevant knowledge of physiology. The NERSC supercomputer "Cori" was named for Gertie Cori, who helped discover some of the core mechanisms in human metabolism during the golden age of metabolic biochemistry.

I think the simplest way to think about it is being able to target specific GPCRs is like being in front of a switchboard with buttons and knobs controlling every critical detail of how human bodies manage and maintain themselves.

A better question would be what don't they enable. Something like 30+% of FDA approved drugs operate along that pathway. Besides for the boring stuff like being able to trigger cancer cells to kill themselves, we could have a molecule you inject into your eye that opens up your vision to be able to see infrared and ultraviolet light(and beyond).