Getting Gas or Diesel in other countries is hit and miss. 3rd world countries the "Gas Station" may be a guy selling it from an old oil drum in his yard. However in more developed countries both seem to be readily available. Most places have had both gas and diesel car/trucks and the need for both is great.
However, be careful with Diesel. New trucks/cars sold in the USA, Canada and much of Europe require Ultra Low Sulfur Diesel(ULSD). When you head south of the boarder it is not readily available. This is one reason the the big Overland rig makes are using gas, ULSD is just not as available as gas.
Before you set out to buy that fancy new Diesel truck, check the countries you plan to visit to see if ULSD is sold.
If you have an old Diesel, it is not an issue. The change took place in the last 10-15 years. So, if you drive a 80-90's Diesel there is not an issue. Most of those trucks will burn any carbon based liquid you want to dump in the tank, from JP8 to veggie oil.