The thing that they don't like is when a story like, has 3 lines saying that MC was male, now is Female, and has NO further lines regarding this. To be more specific, if you remove those 3 lines, and NOTHING changes story wise (Besides Yuri, most GB use GB to get Yuri in) then that's what people have issues with. Its like those random plot points placed in manga, that have big implications, and then are NEVER mentioned or used again for the rest of the story.
No gender crisis, no 'oh I didn't know that happened to girls' no thought process belonging to original gender (mc who was male for like 20 years suddenly just thinking 100% like a female the moment when become girl) and no sudden issues with like morality on if they should or should not do X because they used to be a certain gender.
basically nothing added to it beyond 'oh hey, I used to be male, now am suddenly female, that's all, bye bye!'
not even an in world explanation, like say, the process of reincarnation solved the issue, or that they lived and have the memories of being said gender because they were raised again as a baby as said gender.
Rule of thumb: if you can remove the initial sentences stating mc got Gender Bent, and just say mc was that gender all along, and nothing changes plot wise (ignoring Yuri/Yaoi) then that's GB used poorly
cant blame them all though, some might have started with GB with the intentions of making it important, and ended up drifting too far away from the original idea