A surviving GMO put aside, how far could you go in cross-species gen editing?
Is it possible for example to introduce plant genes to human DNA and vice versa?
Question: from where is gen editing limited in this context? or is it unlimited?
So, I'll assume that you are talking about gene editing in a laboratory (e.g. using a technique like CRISPR).
Theoretically, there are almost no limits to what you can do - or at least try. Fundamentally DNA is the same in all living (and non-living) things on earth, so you can absolutely take a gene from a plant and "paste" it into a human cell/genome (with some additional work needed to make the pasting work). You probably even go further and try to isolate whole chromosomes from one organism and (literally) inject them into cells of another one.
The problem with this radical approach to gene editing is, that it probably won't do much (good). A human cell likely doesn't recognise a plant gene as a gene and so just ignores it (it might still cause indirect problems for the cell though) and if you start making very drastic changes like adding chromosomes the chances that your cells will just die are super high.
This means that in practice you either need to (heavily) modify a gene you want to insert to match the structure and 'expectations' of the target organism - you basically have to make a humanised version of a given plant gene - or you can only use genes from similar enough organisms: as a general rule genes from the same kingdom (bacteria, archaea, plants, fungi, animals) are likely to retain some degree of function in another species (but how much exactly or even a general effect is almost impossible to predict).