When Caesar invaded Britain the second time, he was described to have levied 2-6K Gallic cavalry to augment the Roman cavalry. I've seen seven sources and all use the word levy. Wikipedia says request. I have not seen the particular source that it cites.
In the pre-Marian reform days, when troops were levied from either Rome proper or allies, it was grabbing a bunch of citizen farmers and aristocrats, give some boot camp training, and then putting them under a banner. They did not get paid and often looted in lieu of the money they could be making on the farms.
This is post reform and the maniple is gone, but I'm wondering if Caesar actually paid the Gauls for their help. Levied seems to implied he basically forced the tribes under his control to contribute. These men could be farming, hunting, logging, raising cattle, or pigging out at whatever constitutes as barbeque back then instead of going to Britain.
Also, in the last three years of his governorship (much later than this), he pretty much paid his own legions with a mixture of coin and IOUs. So like one centurion got 3 IOUs and 1 coin for every 4 he was supposed to be paid. While the IOUs were ultimately made good on after he marched on Rome, it leaves me wondering if he just ordered the Gauls around without pay.
If these Gauls were treated the way Rome treated allies in the day before paid soldiers rather than being treated as mercenaries, it is no wonder that the Gauls would revolt in a year.