A gallon, if the reduction is 4 to 1 to 1, which means, 4 parts paint, 1 part catalyst (hardener) and 1 part reducer will give you 6 sprayable US Quarts. Assuming that it takes .75 quarts to fill a pot and 2 pots per pass around the truck...you would have enough for 3 coats of paint and being Black (which in general does cover fairly well) you should have enough to paint your truck. If they sell this paints in Quarts as well, it may be wise to purchase 2 extra Quarts and whatever additional hardner you may need...reducer is often sold in Gallons and if that's the case, you should have enough reducer. The reason I suggest having more paint on hand is that there is nothing worse than running out of paint 2/3's of a way through a paint job. If you do, you may need to start let what you have painted so far, cure, re-sand the Truck and apply more paint. If you have it on hand, what you don't use I'm sure you could return or keep it for future projects (it's Black, you always need Black paint)...and it's John Deere Black...chances are it's not as pricey as PPG's Dupont's or other major manufacturer's brand of automotive refinish Black paints.
Hope this helps, if you need any other tips or advice, feel free to ask, always happy to help out.