It would be GMO if genes from a bacteria or other plant species like cucumber were used to create feminized cannabis seeds. GMO is about using DNA code (genes) from one species in another to genetically modify the latter so it can do something new or does something less, i.e. transgenesis. Can't type DNA code like a programmer (yet), so have to borrow building blocks from others organisms, then shoot them into the target organism using a gene gun or bacteria in a lab.
While that may sound nuts and scary, it works because all organisms already share a lot of their DNA. The DNA sequence of a set of genes responsible for creating protein X in humans is exactly the same in plants and animals where the same protein is created. The protein coding DNA of a human is for example for roughly 50% the same in a banana plant. That allows scientists to isolate parts of code that belong together and have specific spots (before and after it) where new genes can be added without making a mess of it.
Creating feminized seeds obviously does not require the genome of cannabis to be modified, it is already (too) good at creating fem seeds.