In the latter half of the 21st century, Gleaners were a group of subversives, who with their distaste for the Global Corporate Congress chose to live off the grid, eking out an existence away from the major cities, cultivating organic crops. The group seen in "Waning Minutes" hid up in the Rocky Mountains, cultivating their meager crops in canyons or underground caves.
They are called "Gleaners" because they "glean" a meager subsistence-level lifestyle off the grid. As their leader stressed, while their intentions were to remain independent, their chief survival strategy was not to oppose the Corporate Congress, but to remain so small that the GCC would deem them too insignificant to be a threat. Therefore they intentionally kept their numbers low, rather than expand their settlements in cave systems. It was still a risky existence, as the very crops they grew were illegal: years earlier, the corporations had proliferated genetically modified crops which produced no seeds while crowding out organic strains, meaning that ultimately farmers could not keep the seeds their own plants produced, but had to buy new seeds year after year from the corporations. Afterwards, the corporations passed laws making it illegal to cultivate organic, replenishable crops.
Unknown to most in the settlement that Cameron visited, the GCC was already aware of their existence, yet besides their other precautions (keeping their numbers low and relatively unarmed so they would be deemed insignificant) their leaders sent food shipments to the GCC. Organically grown crops didn't have all of the taste bred out of them yet like genetically modified crops, and were hypocritically considered a black market treat by corporate executives. Therefore, the leaders of the gleaners essentially bribed off the GCC with regular shipments of their surplus organic crops.
Kagame pointed out that the Gleaners did not really oppose the Global Corporate Congress, as Liber8 did. At best, they had removed themselves from the outside world but were going to make no effort to change it either - and in reality, they were actually bribing the GCC with rare organic foods, and indirectly supporting them. Kagame therefore tipped off the GCC that he had been at the Gleaner compound (by smearing some of his blood into a shipment of apples, which was picked up by DNA scanners). This caused the GCC to bomb the Gleaner compound and kill all within it. In Kagame's black and white view, the Gleaners weren't really helping in the fight against the corporations, and were actually detrimental to Liber8, because they provided a non-violent alternative for potential anti-corporate recruits who were nervous about the level of revolutionary violence advocated by Liber8 (specifically, deterring Sonya from joining Liber8).