CYL is a keycap profile produced by GMK.
CYL was originally known as "Cherry" profile, since GMK acquired to the tooling for these keycaps from Cherry.
For a long time, this was the only profile GMK used to manufacture keycaps, which lead to keyboard enthusiasts simply referring to these as "GMK" keycaps.
When GMK started producing a new keycap profile (MTNU) they retroactively named this profile CYL to distinguish it by its cylindrical shape.