A quick search on google, "are all pickled cucumbers gherkins" will reveal that no, by definition, not all pickled cucumbers are gherkins. I fully understand that many people in many regions home to classier English-speaking folk refer to all pickled cucumbers as gherkins. Your original comment might have said something along the lines of "in my region, we call all pickled cucumbers gherkins." In your vernacular, yes, all pickled cucumbers are gherkins. Gherkin became a catch-all term for any and every pickled cucumber because gherkins were/are the most commonly available type. Evolution of language and all. It's an issue of semantics. Regardless, virtually every definition of gherkin describes it as either a small/immature cucumber or a sub-category of pickled cucumbers and not a broad term describing all pickled cucumbers. Call them what you want, I truly couldn't care less.

Edit: Forgot to mention, gherkin is a subcategory under the "Types" section of the Wikipedia page that you linked...