Indeed, Wheat = Pszenica and polish Zboże can be translated as either cereal or grain, depending on context(for example, in scientific context, zboża rzekome like gryka are understood as not being part of zboża and therefore cereal would be better translation, but in common speech most people understand zboża to mean grain).
"Pseudocereals are non-grasses that are used in much the same way as cereals (true cereals are grasses). Their seed can be ground into flour and otherwise used as cereals. Examples of pseudocereals are amaranth (Love-lies-bleeding, red amaranth, Prince-of-Wales-feather), quinoa, and buckwheat."
In polish, zboża rzekome.
And yes, as far as I know, Gryka = buckwheat.
English wiktionary – while the Polish coverage is not the best, it's getting better and it's the only resource I can think of, that has this kind of informations about Polish in English.
But I don't know, maybe there is something else – generally, I use Polish language resources, so I'm not the best informed person about English ones. ;) I would suggest post about it on Polish forum, as other Polish learners might have some ideas I'm missing.