In general GMOs do not further the science of genetics per se; rather they build on and exemplify already-developed genetic knowledge.
For example, they clearly demonstrate that the codes embedded in DNA are not species-specific: a gene that produces a certain protein in any organism (e.g., a bacterium) produces the same protein in a plant. It also demonstrates that any gene has, and needs, a turn-on switch (termed a promoter) as part of the DNA that is “read” before the gene. This can be tissue specific or specific to some developmental stage, such as in Golden Rice where the carotenoid precursor on vitamin A is produced in the inner tissue (the endosperm) of the rice grain.
The insertion of a DNA sequence has been found to have no significant effect on the expression of other characteristics, provided that the insertion site is not inside some other gene; often multiple insertions have to be performed to get one in an appropriate location. If correctly located, such genetic engineering has less effect on surrounding genes that conventional breeding, where genes are mixed in a random assortment.
The new gene-editing techniques, such as CRISPR, work equally well in plants, and such techniques will lead to more precise gene-editing in plants in the future.