How might one trip a breaker from a GFCI outlet?
You want to trip the overcurrent detector (breaker) serving a GFCI outlet. 

No. Don't do it.

What you're looking for is so similar to the other question that it really is a duplicate.  It is wrong for all the reasons that one is wrong (and not insane in a certain industrial setting for the reasons I describe in my answer there). 

The presence of GFCI is irrelevant.  (though of course if you use the hare-brained scheme in the other question, you have a good chance of frying the GFCI's innards; it will cheerfully allow the overcurrent if balanced, but its detection circuit will fail before the breaker will). 

Your parameters for "safety" are too narrow

You are ignoring the likelihood of failing a circuit due to the overload opening a wire connection, or worse, setting the stage for a future arc-fault problem that could burn down the house the next time you load the circuit within reason. 

Then there's the matter of arc flash right there in your face. 

Plug in a radio, and snap breakers

The way you solve this is by plugging in a radio and snapping breakers off until the radio stops.  Pretty simple.  If you don't have a radio, a vacuum cleaner will also do.  

You could also plug in two 1500W heater-fans and wait 20 minutes. That will trip any 15A or 20A circuit eventually. But again you are risking the same kind of wiring failures discussed in "too narrow" above.