Outside of a laboratory there are no chickens that are genetically modified or the offsring of GM ancestors.
Genetically Modified meaning that they have not be produced with gene splicing, cloneing, assembling various body parts (frakenstiened), nuking or any other extraordinary means.
Broiler chickens are, in a simplified manner of understanding it, the product of four seperate families. If you will, the grandparents are each of a seperate family unrelated to one another. The grandparents are in the truest sense of the word "breeds" unto themselves. These families meet (actually excell at the critera for identification as a breed in everyway. Except that they are not recognized by a name in the general public.
The end product if this process, in this case the Ross 308s, are just simple crossbreeds. Nothing more.
The term "Hybrid" was applied to them decades ago as an advertising gimmic.