Let's take corn (maize) for example.
In the USA, where corn is grown in abundance, the grains are normally ground down into flour or cornmeal. Whereas, here in the UK we would recognize the bright yellow kernels as sweetcorn, and we would eat them as a vegetable. In both cases, of course, the seed itself is gluten free, however we classify it.
Just to complicate matters further, cornflour in the UK is gluten free, but the same product is known as cornstarch in the US and can be mixed with gluten containing ingredients so is off limits.