How do I apply a gravity/physics effect to these window curtains to make them look like they are hanging from the curtain rods at the very top?
I’m not animating them, I just want to make them hang and bake the effect to the objects for a single frame render.
Each curtain is it’s own individual object and have the wave effect modifier applied to them.
Have you tried using cloth sim?
This videos seems to have everything you need
Obviously, you only want to do one set of the curtains, and then apply the cloth sim, and duplicate. It’s going to add a lot to your poly count…
Does that work in conjunction with the wave modifier?
Not sure, but I doubt it. Make a backup copy, apply the modifier and then perform cloth sim. Then you can go back to the wave modifier if needed.
I’m using 2.79
Does that version have the cloth sim?
How do I use the cloth sim?
Yes 2.79 is fine!
Explaining how to use it doesn’t really fit in one message. Watch the tutorial peter18 posted, that seems to be exactly what you need. 
Thank you!