I've done this with three separate gpus, with no problems on my 15kHz pvm. One was an nvidia laptop gpu, one a nvidia desktop gpu, and one a AMD desktop gpu. I know people have said they've had problems, but in my experience as long as the card has analog output, it will work. Most recently did it with a GTX 950. These days the analog output is through DVI rather than VGA in almost all cards. Make sure at least one DVI port is DVI-I rather than just DVI-D; you can check visually by ensuring that port has holes for the red pins at the right of this diagram. As far as I know none of the Nvidia 10XX series has analog output, but the 9XX series does.
Then you'll need a DVI to VGA adapter (tiny thing, might even come with your graphics card), and a VGA to 5 BNC connector (R, G, B, Hsync, Vsync) which costs about 10 dollars. Do not get a 4 BNC connector, as this will only have one sync signal, and you need them both. You just get a 2 to 1 bnc combiner/splitter, and put the two sync cables into it.
Now you have the 4 bnc connectors to connect to your monitor. Should be no problems at all running at 720p or 480p, though I have no first-hand experience with this. If you want to run in 240p, you might need an extra step.
If you're lucky, your graphics card will be able to create a custom resolution of 240 by whatever (it's actually good to use a huge number; I use 240x2560 but some use double that), and it will work no problem (this happened for me on my laptop gpu). However you'll probably want to use the
CRU tool, and set up the custom resolution through that utility. You might have to play around with the different values for front porch, sync width, etc. until you get a picture that syncs; it can be a bit finicky.
Anyway, if you don't need to do 240p, it should be a breeze; I imagine you could even do it through intel integrated graphics (I haven't gotten this to work with 240p, but it should handle standard resolutions just fine).