Obviously everyone was not born into the gentry. It was a pool that had to be continually refilled. There was a practice in England, if not other countries, known as distraint for knighthood where the crown compelled wealthy persons to be knighted. It began in 1224 under Henry II. Anyone who possessed lands earning more than £20 per year was obligated to become a knight or pay a hefty penalty. Under Edward II, the threshold was raised to £40 where it remained throughout the era of knightly service. When you consider that the yearly rent for a cottage in the fourteenth-century was only 5 shillings (one-quarter of a pound), £40 was certainly not setting the bar very low.
[Source: “Romance and the Gentry in Late Medieval England”, Johnston, Michael, Oxford Press.]