Gal Cleaning is definitely the number one manga I've read that got the biggest backlash from its readers. Why fans where unhappy? Well...
The first part of the manga, meaning chapter 1-8.5, is nice fluffy stuff about a shy guy called Kaji who somehow ends up cleaning a classmate gal's house. We get the usual lighthearted romcom antics and some simple cleaning tips to go with it. All is fine and wholesome, and the readers prepare for the inevitable other fluffiness and romantic developments.
But then we get to chapter 8.6, aka the start of the mangaka's spiral into madness. See these content warning tags?
Well, all of that stuff only happens after ch 8.6. Speaking of which, the chapter naming is ridiculous: after 8 came 8.5, 8.6, 8.7, 8.8, 8.9... 8.91... up till 8.99.
Now, the first couple of chapters after 8.5 are still more or less okay. It's a joke that gets dragged out way too long, but it's not unreadable until 8.94, at which point we suddenly get several chapters of sexual abuse in our fluffy gal manga. Oh, and that sexual abuse is forgotten basically immediately afterwards.
After that we get some intolerable misunderstandings borne of interactions no human would ever have and a lame ending.
If you're wondering why the author went this road: so is everyone who read this series. The most prevalent theory is that vol 1 didn't sell well and the author had a breakdown or decided to troll fans. No editor should have allowed this though, and it's a kick in the nuts of the few people who actually liked the author.
Read up till chapter 8 or 8.5, then drop it. Alternatively, look for better gal-themed manga: there's enough of those by now.
Personal recs: Dosanko Gyaru wa Namaramenkoi, Gal Gohan, Kumanishi Bijutsu-Bu: Rafusuke Senpai, or even Anjou-san