Gaul was a region in Europe that existed around the time of the Roman Empire.
It comprised of 21st century France, Belgium, Luxembourg, part of Switzerland, Northern Italy and bits of Germany and the Netherlands. (Combat Magicks)
In 64 AD, Flavius Giscard campaigned in Gaul while the First Doctor, Ian Chesterton, Barbara Wright and Vicki stayed at his villa. (“The Slave Traders”)
In 451 AD, Attila the Hun led an army against Rome. The Thirteenth Doctor and Team TARDIS met one of his commanders, Bleda, between Orléans and Chalons.
Asterix the Gaul from a comic strip by Goscinny and Uderzo was from Gaul. (Combat Magicks)
In an alternate timeline where all of history occurred at once after River Song refused to kill the Eleventh Doctor, Holy Roman Emperor Winston Churchill went to Gaul to speak with Cleopatra. (The Wedding of River Song)