12 AWG to GFCI to 14 AWG. OK?

Is 12 AWG to GFCI to 14 AWG OK on 20 amp breaker?

It is permissible to combine a 12 AWG cable to a 14 AWG cable, providing the amperage of the circuit does not exceed 15 amp.

It is a serious fire hazard to run 20 amps through 14 AWG cable!

NO.
14 gauge wire is only capable of safely carrying 15 amps of electrical current. Therefore it must be protected by a 15 amp or smaller breaker.

Although it can be done if you follow the proper circuit ampacity guidelines, it's not a good idea to do it. Let say that you start with 12 ga. Wire at the panel, the after a gfci you drop down to 14 ga. Anyone opening up the panel would make the false assumption that it is a true 20 amp circuit and provide overload protection based on the wire size. That immediately creates a potential hazard.

The answer is still no.

The question was
Is 12 AWG to GFCI to 14 AWG OK on 20 amp breaker?

OP revised the question in second post.

Also, if you take certain circuits, like kitchen, laundry, bath, and reduce from a 20A breaker to a 15A not only is this foolish because you are reducing the capacity of the circuit needlessly, you might also be creating a code violation. Receptacle circuits in these areas are required to be on a 20A circuit.

Bottom line is there is really no vaild reason to reduce from #12 to #14 on almost any circuit.