The barrel is a unit of volume used in the oil industry with the symbol bbl.
Oil is more commonly expressed in terms of cubic metres (m³) but in the US and in the economic world (mainly because such a large percentage of the world's oil is shipped to the US) as barrels.
The measurement of the barrel originated in the US at the first oil well in Pennsylvania. There were so many variants of barrel size (including those used for wine and other goods) that the distrust became so prevalent a central and standard barrel size was created. This was finally established in 1872 as 42 US gallons.
The US gill is a unit of volume in the US customary unit system and uses the symbol gi.
It represents 1/4 of a pint - so in this instance, it is 1/4 of US pint - therefore measures exactly 118.29411825 ml. It's also equal to 8 tablespoons, 24 teaspoons and 5/6 imperial gills.
The gill has been mentioned a number of times in popular literature; including in Treasure Island, Animal Farm and in Dan Simmon's 'The Terror' - and all references are pointing to the use of the gill in terms of alcohol measures or servings.