Netlify also provides support for forms which could be used to accomplish this.
Can't speak to the GDPR aspect, but I don't imagine it would be much of an issue for just saving and displaying comments.
I wrote a short post how to set it up on heroku to avoid GitHub quotas problem: https://muffinman.io/running-staticman-on-heroku/
I used Staticman for a couple of years and it worked fine. But GitHub changed their API policy which broke Staticman.
Also, spam is really hard to handle with this. I had to unplug Staticman from my site because of it.
Can you clarify this / have any links? Thanks
EDIT: Ah, just tried the demo (now broken). It looks like it was allowing internet random's to post to GitHub under a single account owned by the site or the project, and I totally get why GitHub is going to pull that ASAP.