I'm definitely not a fan of posting most of a story online and then asking readers to pay for the ending. it's disingenuous.
One of my parent's friends has a long history of working in direct marketing (also called multi-layer marketing or pyramid schemes, you know, when people try to sell their friends a bunch of stuff that usually doesn't work?). Anyway, this "friend" only comes calling when she has something new to sell. Then she'll be super friendly and nice. And because she's a friend, we all feel like we need to support her and spend tons of money buying stuff we don't need. Once she's made a sale, she disappears.
Authors who use sites like GA solely for advertising purposes feels like this family friend. GA is not an advertising platform, it's a place for people to share stories, talk about stories, and form a community. To take advantage of that is wrong. And I say that as someone who has posted free stories on GA, and who is now also publishing ebooks. Yeah, I would love it if my GA readers bought my books, but I'm not going to hound them around the site until they do. If people want to buy my books, they'll reach out to me for the info themselves.