It isn't that you cannot mine scrypt coins on Prohashing with a GPU rig: it is REALLY inefficient. I could go setup my gaming rig in my basement, turn on CCminer point it here, and draw 500-600 watts of power for about 500khash/sec, or I can use my ASIC miners, and get 15-18Mh/s (30+ times faster).
Further, Regarding ROI (return on investment) if you are building a GPU rig, with a couple of RX580's, assuming a new build with an i5 and such, you are spending 1500 to 2000 to build it. Depending on what you mine, you may be a year before you recoup your investment with those lower hashrates on scrypt. This can get you well towards an antminer D3 or L3+ which each will pay for themselves in just a few months.
However, a GPU rig could mine other algorithm coin Etherum (ethash), or Vertcoin (Lyra2) where the ASIC cannot. Those coins are doing quite well too. Using vertcoin for an example, a 1080ti rig could mine that quite well, and yield a coin a day (or so) at $6 a coin. Return on investment on a $2000 computer in about a year. (assuming difficulty doesn't change).
If you are hellbent on GPU mining, do some google searches on which algorithms you want to mine. Then which coins are supported on those algo's. Some algo's do not mine on AMD, and some mine on Nvidia. Depends on the program used to mine. Then use a mining calculator (coinwarz and others), and determine if mining that coin is financially viable. Read up on the coin, stability in the market, value, fluctuations, and if the coin interests you. Take a hunch if you think it is something that will grow. Does the development team put effort into it expanding (adding exchanges, etc). Then mine it for a short term as a proof of concept, and see if your numbers hold up. Then mine it. Then always keep an eye out for newer ways to mine more efficiently.
Use google. start reading. Daily. I was green like you 3 weeks ago. I learned it all using google. Take your time, do you research first so your hardware is mining as efficiently as possible. We cannot spoon feed you everything, but we can try to help.
Good luck
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