We don’t know of any legitimate GM Marijuana in development or in existence. Any rumored involvement by CBI or seed companies is “urban legend.”
Cannabis has definitely been genetically modified for the underground and “medical” markets, but not using the modern methods that get called “GMO”. The modification of the genetics of marijuana achieved using a combination of traditional breeding techniques and clumsy, “old-school” techniques like chemical mutagenesis and induced polyploidy. In other words, various enterprising people used toxic chemicals to cause mutations or used Colchicine to induce the plants to double the number of chromosomes in every cell. Some of these plants grew better and/or made more THC. That is why modern marijuana is so much more potent.
There is a long, completely unregulated history of using these techniques to improve legal crops, but such methods are far more likely to cause undesired and undetected additional changes in the plant’s genetics. Modern genetic engineering is a much safer option.
The Cannabis modification occurred completely outside of any mainstream company or regulatory scheme. In other words, Monsanto has nothing to do with this. There is one very legitimate potential application of precise genetic engineering with Cannabis. There are chemicals other than THC in marijuana that are very helpful for some forms of epilepsy. If the genes for THC production could be turned off, the resulting plants could be a good source of that therapeutic chemical.