It is wrong to think that nobody in mainland China tolerates gay lifestyle but based on experience, those tolerant/progressive mainland Chinese people would be few and far between.
The thing is even straight/hetero sexual activities in China still not at all a popular dinner table topic of conversation let alone what gay people do in the bedroom. That is still very much a taboo I'm afraid. A typical mainland Chinese family never talks about their individual intimate sexual activities with their family members.
What I can tell you is that many mainland Chinese families with an openly gay son/daughter accept them for who they are. The neighbors, well, that's another matter. Maybe they will, maybe not.
Look, people will think what they want to think wherever. The main thing you should consider is your own happiness and also give some thought on your family's reputation in your community. I can tell you that there are other families who have "lost face" with their friends and community members after they found out their child is gay. You must weigh the pros and cons.
I remember a Chinese grandma who posted a video on youtube that went viral asking her community to accept her grandson for being gay. Although that was a very heartwarming and thoughtful thing to do, it also tell us about what local Chinese community thinks about gays.
As I said, people will think what they want to think. If you're too miserable being in the closet and you think coming out is really significant and important to you then by all means come out. Just be strong just in case there are some negative blow-backs.
The actual coming is easier. Just ask your family members (especially your parents) for a private moment and that you have something important to tell them. And just as gently as you can tell them that you love them and that you are gay.
I hope this helps in anyway and good luck.