The GL class was the first PRR class of what we today call a "hopper". At the time of their creation there was no "hopper" name, so they were thought of as gondolas, and so they wound up with the "G" class. Prior to this, coal and coke was moved via box cars and regular gondolas, both of which were hand-emptied. The GL was an all steel, fish-belly design hopper that was built starting in 1898 up through 1904 by a wide variety of manufacturers. A total of 20,138 were made, so they represent a significant percentage of hopper cars in the era of my interest.
These are two-bay hoppers that measured 35'5" end sill to end sill, and had an overall width of 10'1-1/2". They weighed 40,000 pounds and could carry a load of up to 100,000 pounds, or a volume of 1,897cuft. The original cars had vertical brake shafts, but starting in 1911, they were changed to horizontal brake wheels.