120V outlet from 240V load center?
Get that bond screw pulled, and get a PK7GTA in there, then you can add a circuit here

The first step is to sort out this morass of a spa panel box.  The good news is that it's a QO612L100RB, which means that you can get just about any sort of breaker you'd ever want to put in there.  The bad news is that the original installer butchered the neutral/ground handling, leaving the bond screw in and spamming all the ground and neutral wires onto one bar, so we have to fix that before we can start adding circuits.

You'll need a PK7GTA ground bar and a screwdriver to fix this, in addition to the breaker for the deck outlet.  First, turn off power at the feeder breaker for this panel.  Second, you'll need to remove the green screw next to the neutral lug; this removes the bonding screw from the panel, and separates ground from neutral here.  Third, you'll need to install the PK7GTA in the panel, screwing it to the raised holes bottom left or bottom right.  Finally, you'll need to take the two bare ground wires out of the neutral bar and connect them to the new ground bar instead.

Once this is done, you can add your branch circuit as normal. The decision as to whether to use a GFCI receptacle with a regular breaker or a GFCI breaker with regular receptacles is yours; the NEMA 3R enclosure of the panel might provide marginally better protection for a GFCI than a receptacle box would, but GFCI breakers are much more expensive than even a weather-resistant GFCI receptacle, so there is that as well.  Don't forget to size the breaker appropriately for the wiring you are running for the new circuit (15A for 14AWG, 20A for 12AWG), use properly outdoor-rated wire (UF instead of NM, or THWN in conduit if you are going the conduit route) and boxes (with while-in-use covers), and torque all breaker and loadcenter screws/lugs to specification with an inch-pound torque screwdriver (as per 2017 NEC 110.14(D))!