Wiring for outdoor GFCI receptacle in my garden
Wrong conduit for the job

First off, you are looking at the wrong kind of conduit for this task.  NEC 348.12 point 6 prohibits the use of FMC underground:


  (6) Underground or embedded in poured concrete or aggregate


Given your desired trench depth, you want to use rigid metal conduit (RMC) instead, as even liquidtight flexible metal conduit (LFMC), which can be buried, requires 18" of topcover.

You don't need wires that fat, either

Second, your 8AWG wires are quite a bit too large; for a general-use receptacle at a mere 85' distance, 12AWG will suffice.  (Rule of thumb: if the one-way length of the circuit is longer than the nominal voltage, then you start need to worrying about voltage drop.)

Other tip: put the GFCI at the house

Another tip here is to put the GFCI inside, at the house, where it is better sheltered from the weather.  While a good weatherproof box and while-in-use cover will do their darnest to keep rain out, they're still not as good as the roof of your house at it.  Also, don't forget to fit the bottom of your receptacle box with a weep hole (1/8" is fine) to both let water out and keep air pressure inside even.