I want to install outlet on existing junction box
That box has plenty of room, but you need to be strategic in where you place the wire nuts.  There is a hollow in the middle of the box, and they should be pushed back down there.  It also helps to use a less bulky model of wire nut.  Ideal brand is popular among electricians.

As far as GFCI, don't put a GFCI here.  It's outdoors and the GFCI will quickly corrode and fail.   

GFCI devices are able to protect a downline circuit, e.g. A GFCI circuit breaker does exactly that.   So follow this circuit back toward the ser ice panel.  If it makes a stop at an indoor receptacle, fit the GFCI there and place this downline on its LOAD terminals.  Many people use those terminals accidentally because they don't know any better; use them deliberately.  Or you can use a GFCI breaker. 

Then, fit a plain receptacle here and place "GFCI Protected" stickers here and on any other receptacles that are now so protected. Those are included with the GFCI.   

Lastly, fit an outdoor "in use" "waterproof" cover.  They don't work, but they're required by Code.  The GFCI inside the house will do the protecting of all the wire in this box including the LINE side wiring  (which a GFCI out here can't protect).