No, the disciples weren't "practicing gleaning, as described in Deuteronomy, Leviticus, and Ruth?".
Leviticus and Deuteronomy define specific requirements for land owners:
When you reap the harvest of your land, you shall not wholly reap the corners of your field, nor shall you gather the gleanings of your harvest.
And you shall not glean your vineyard, nor shall you gather every grape of your vineyard; you shall leave them for the poor and the stranger …
— Leviticus 19:9–10
When you gather the grapes of your vineyard, you shall not glean it afterward; it shall be for the stranger, the fatherless, and the widow.
— Deuteronomy 24:21
At harvest time, some of the crops should be left behind for the poor to gather for themselves, which is clearly what Ruth was doing:
So she stayed close by the young women of Boaz, to glean until the end of barley harvest and wheat harvest; and she dwelt with her mother-in-law.
— Ruth 2:23
But, this is not what the disciples were doing here:
At that time Jesus went through the grainfields on the Sabbath.
And His disciples were hungry, and began to pluck heads of grain and to eat.
— Matthew 12:1
Now it happened that He went through the grainfields on the Sabbath; and as they went His disciples began to pluck the heads of grain.
— Mark 2:23
Now it happened on the second Sabbath after the first that He went through the grainfields.
And His disciples plucked the heads of grain and ate them, rubbing them in their hands.
— Luke 6:1
The disciples are not harvesting crops left for the poor, but something completely different:
When you come into your neighbor’s vineyard, you may eat your fill of grapes at your pleasure, but you shall not put any in your container.
When you come into your neighbor’s standing grain, you may pluck the heads with your hand, but you shall not use a sickle on your neighbor’s standing grain.
— Deuteronomy 23:24–25
This law is not about providing food to the poor at harvest time.
Its purpose is to allow people to feed themselves while traveling, which is exactly what the disciples were doing.
They snacked on the food immediately, not taking any of it away with them.
And they plucked the grain and separated the chaff by hand, not using harvesting tools.
They were not gleaning.