We all seem to have this false impression that it is not possible to apply for GC during F1,CPT/OPT. But it is actually very untrue. This rhetoric seems to arise from the fact that very rarely a employer will be willing to sponsor you in those visa status. If ones' employer is willing to sponsor, one can get his GC in F1 visa status same way as in H1B. (My friend got his EB3 GC within 1 and half years during his OPT).

But it is to be noted that it is very rare that employers will agree to file for Green card while one is in F1 status.