Is "gainer" high carb? You can get away with eating just about anything on a ~200km ride. Although when I did a 200k on almost all malto/whey mix, I got nauseous with 30 miles left. Getting the salt mix correct in my mix was never something I mastered.
On one of my 1200km grand randonnees, I'm pretty sure most of my calories came from some form of chocolate milk and Reese's peanut butter cups. I switched it up by drinking cold cappuccino drinks in the mornings. At about day 3 the craving for real food became overwhelming. It was so hot I'm somewhat amazed that I got enough salt.