I found these:
https://www.vice.com/en_us/article/mb8774/i-got-my-penis-zapped-for-better-boners - there are some important things to pay attention, including who may not qualify for this.
https://nypost.com/2018/03/28/men-are-zapping-their-penises-to-treat-erectile-dysfunction/ - it's the NY Post, which isn't the best source, but it's a decent article.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5121537/ - this is about shockwave in general, not the GainsWave.
So it seems, to sort of summarize, that success seems on why you have ED, how many treatments you get (and they are about $500 per treatment, and if you're in the US, insurance doesn't cover them because they aren't FDA approved yet), and some men aren't good candidates. https://gainswave.com/am-i-a-candidate/
It seems there is a clinical trial happening that you might qualify for - https://www.ptcommunity.com/wire/largest-clinical-study-focusing-gainswave-erectile-optimization-approved-begin-enrolling
Exercise and living a healthy lifestyle may give you better results, depending on the cause of your ED.