patented genes that spread to neighboring fields
All genes do. If you are referring to the 'people getting sued' over it thing, look into it further. No one has ever been sued for simply being cross pollinated, and give China's general stance on IP of any kind, I highly doubt any company would have a chance of successfully suing in China.
genes that provide restitence to weed-killers spreading to wild species
To my knowledge there has never been any documented example of the herbicide tolerant gene jumping between GMO crops and weeds. There has, however, been selective pressure on weed populations that has resulted in the emergence of herbicide tolerant lines (by for example having a mutation at the binding site of the enzyme the herbicide targets). The key context here is that, one, this is due to over-reliance on the glyphosate herbicide (the main one of the two herbicides that crops are resistant to) instead of using herbicides of multiple modes of action, two, the problem here is that these weeds will diminish the benefits already provided by herbicide tolerant crops. The ideal would be rotating through multiple modes of action to mitigate resistance, however, due to the benefits of these crops, there has been too much reliance on them, which is why there is now more of a push to diversify the herbicides, although no doubt in the future glyphosate will still be preferred. I also fee it must be said that herbicide resistant weeds predate GMO crops by a few decades; although the case with GMOs is particularly problematic due to the gains that are at risk, this is not a new problem. There's a lot of hatred for the herbicide tolerant crops, and on the surface that makes sense, but I find people rarely have the background context and complete story.
modifications that hinder the production of viable seeds, so the farmers have to buy new GM seed from the producers rather than growing part of their harvest on next year
That doesn't exist outside labs. Anti-GMO people love to talk about that one but they lie. What is out there is hybrid seed, which has been in use since the 30's, which has better yields, more hardy, ect. the first year but subsequent progeny is so genetically variable that it makes economic sense to continue to purchase hybrid seed. Think of it like this, you cross AA with BB to get AB, which could be the best, but when you cross the AB and AB offspring you get AA, AB, and BB, which doesn't work out so well. Corporations didn't do this, genetic engineering didn't do this, its just basic genetics.
I'm sure GM would be welcome in most countries if it was not for the companies producing them. Another thing is that the Chinese are fully capable of developing or buying the technology themselves - so why should they allow in American companies that are only intent on siphoning off as much profit as possible to their share holders?
Well, you're wrong. They are Chinese developed varieties, the rice developed by Huazhong Agricultural University and the corn was developed by Chinese Academy of Agricultural Sciences' Biotechnology Research Institute in Beijing. Contrary to popular believe, more than just corporations are using technology. They're just the only ones able to jump through the scientifically unjustifiable regulations. There's cool GE plants sitting in university labs around the world, but the agriculture and plant science departments just don't have the funding necessary to bring them to market like the big corporations do. And to give your notion that non-corporate GMOs would be welcome another counterpoint, note that China does not accept shipments of the Rainbow papaya, developed by the University of Hawai'i, not a corporation.
The movement against GMOs likes to hide behind anti-corporatism, but so much as scratch the surface and you'll find they are just anti-science. Look at the controversy over Golden Rice developed by an NGO. Look at the opposition to Arctic apples developed by a small company. Look at the vandalism of CSIRO's low GI wheat. Every single genetically engineered crop that gains any notability is opposed. No exceptions. You can't call that opposition to some big company. TFA indicates the government may have decided not to pursue the technology further with so much public controversy and as they have been boosting food production without other means. I don't know when China started to care about Greenpeace and other science denialists (as a plant scientist I put them right with anti-vaxxers and young earth creationists). This is sad and frustrating news.