Gas Range requires 120V 15A yet comes with a Nema 14-50 Plug
A 14-50 receptacle can be a 40A or 50A circuit (i.e. that fuse/breaker). If wired with 8 AWG cable, it can only be 40A.  The reason for this exception is 40A receptacles don't exist, because this chart is big enough already! 

If you have a new stove that comes factory-wired with a 14-50, then they sold you an electric stove.  Edit: That's what you have.  Since the red wires aren't thick enough to support an all-electric range, my guess is only the oven is electric. As you say in comments, it's the broiler. 

How does that work?  NEMA 14-50 supplies everything NEMA 5-15 can possibly supply.  In North America we use 240V AC (on L1 and L2), but with neutral (N) tapped in the middle of the 240V, and safety grounding biased to neutral.  (so each leg of 240V is only 120V from ground, making the power safer if you get bit).  


Common NEMA 5-15 receptacles give L1 (or L2) and N, giving 120V, plus ground of course.   
NEMA 14-50 receptacles give L1 and L2 and N, giving a smorgasboard of 120V or 240V. And ground, of course.  In an RV, the 14-50 feeds a subpanel.


My guess is your appliance doesn't actually need 50A of 240V.  It probably needs 15A or 20A - the manual (the real one) will tell.  However, this puts you in a real pickle because 120/240V split-phase isn't found in kitchens - you'll have to wire a circuit.  The default for a kitchen is a NEMA 14-50 receptacle with either a 40A or 50A (8 or 6 AWG) circuit.  

If it can use a 20A circuit (NEMA 14-20) you can run that with 12 AWG wire and save a bit of coin, but no future range will be able to use that connection, because the standard for ranges is 40A or 50A (NEMA 14-50).  Also, building codes may require a NEMA 14-50 to power any future owner's stove.  I realize I'm spending your money, but even if I was spending my money I would pull 6/3 cable and the 14-50 receptacle, so I'm future-proof and don't have to worry about building codes.  The cost difference for the cable is small compared with the work/labor needed to pull it through the walls.  Not a job you want to do twice. 

In this discussion we disregard the obsolete and dangerous NEMA 10, which is a throwback to the days before grounding. They don't force people with older homes to rewire, so appliance makers provide a scheme to connect new appliances to NEMA 10. This scheme is dangerous and has killed.  It's also unnecessary: rules changes make it easier than ever to change a NEMA 10 to 14: either retrofit a separate ground wire; or fit a GFCI breaker and a "No Equipment Ground" sticker. Easy peasy. 

Speaking of that... your photo shows a strap between neutral and ground. That strap is only used in the NEMA 10 scheme and should be removed for any wiring scheme (NEMA 14, NEMA 5) that provides a real ground wire. 

Now if you just don't care about the broiler, you can attach a NEMA 5-15 cord to the black, white and ground wires and you're done.  Do remove the neutral-ground strap!