As several others have noted, Carlin's source is almost certainly an episode in the fifth book of Caesar's Commentaries on the Gallic War, in which a Gallic chieftain calls an armed council. Caesar comments:

"This, in the practice of the Gauls, marks the beginning of a war; and by a general law all grown men are accustomed to assemble at it in arms, while the one who comes last to the assembly is put to death with every kind of torture in sight of the host. " (5.56)

It is likely (though of course impossible to prove) that something like what Caesar describes actually occurred. But it is difficult to say whether such executions were really customary. As witnessed by the proto-ethnographic asides that punctuate the Commentaries, Caesar was interested (or at least thought his audiences back in Rome would be interested) in the customs of the Gallic tribes. To accentuate his own achievement in defeating and subjugating said Gallic tribes, however, he had a clear interest in accentuating their "savage" / un-Roman character. So it would be perfectly in keeping with his authorial purpose to play up a rumor or isolated incident of torture into a barbarian custom.

The only source I know that might provide a sort of control on Caesar's description is the fourth book of Strabo's Geography. Through he wrote decades after the conquest of Gaul, Strabo drew on the (now lost) work of Posidonius, a Greek polymath who probably visited Gaul a few decades before Caesar's invasion. Strabo mentions the important role of human sacrifice in pre-Roman Gallic culture, and provides various lurid examples:

"They used to strike a human being, whom they had devoted to death, in the back with a saber, and then divine from his death-struggle....We are told of still other kinds of human sacrifices; for example, they would shoot victims to death with arrows, or impale them in the temples, or, having devised a colossus of straw and wood, throw into the colossus cattle and wild animals of all sorts and human beings, and then make a burnt-offering of the whole thing." (4.4.5)

Perhaps the "custom" of killing the last man to arrive at an assembly was such a ritual murder, reported to and by Caesar in a more or less garbled fashion.

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