In my opinion 1/2 gallon of gasoline in 26 gallons of diesel will not present any problems. You could add a bottle of any lubricating diesel additive if you want as it couldn't hurt.
Years ago, adding some gasoline to diesel was considered a valid fuel treatment in the winter. Can't remember the ratio but it was a lot higher than .5 gal in 26.not paying attention I pumped a half of gallon of 93 in my ecodiesel. My tank was empty and I filled the rest with diesel. Any helpful advice would be greatly appreciated