It depends on your definition of gleaning, and what time of year they were doing it. Gleaning is supposed to be done by the poor after the harvesters have gone through the field and gathered in the harvest. From wikipedia:
According to the Holiness Code and the Deuteronomic Code of the Torah,
farmers should leave the corners of their fields unharvested, and they
should not attempt to pick up that which was dropped or harvest any
left-overs that had been forgotten when they had harvested the
majority of a field. On one of the two occasions that this is
mentioned by the Holiness Code, it adds that, in vineyards, some
grapes should be left ungathered,
How much is gleaning? Is only a few heads of grain gleaning? Jesus's disciples were definitely not out in the fields with baskets scrounging every last head of grain like Ruth was. They probably didn't even break stride as they walked and plucked a few standing heads of wheat or barley by the roadside. You'd have to be pretty hungry to want to eat more than a few heads of raw unground wheat or barley; it's just not that tasty, especially without salt. Ripe grain's also pretty hard on the teeth to chew.
Was it after the harvest? The scriptures don't say, so there's no way of knowing. The grain was probably ripe, because the Bible says they rubbed the heads in their hands to separate the grains from the stalk and the beard. Was plucking heads of grain before the harvest considered gleaning? We don't know that either. The Bible never actually sets down the rules for gleaning, just the rules that harvesters have to follow so that some produce is left for gleaners. The Pharisees objected not because it was stealing, but because it was "work" on the Sabbath. It's a good chance they did consider it gleaning. They probably would have accused the disciples of stealing too if they thought they weren't entitled to pluck the grain on other days of the week either. Wikipedia assumes too much, there's no way to be definitive; none of the synoptics call it gleaning.