Learn Finnish: Eikö ravintolassa ole gluteenitonta ruokaa? - Isn't there gluten-free food in the restaurant?

First, you add the suffix -ton, which means "without" or "-less" (like the English "-free"). This makes the base adjective gluteeniton (gluten-free). Because it describes the partitive noun ruokaa, the adjective must also be put into the partitive case by adding -ta, resulting in gluteenitonta.

How do you form the partitive case for adjectives ending in -ton or -tön?

It is very straightforward! Unlike some other grammatical cases that require a stem change (where -ton changes to -ttoma-), for the partitive case you just add -ta or -tä directly to the basic dictionary word. So, gluteeniton simply becomes gluteenitonta.

Why is the food (gluteenitonta ruokaa) in the partitive case here?

There are two reasons that overlap here. First, ruoka (food) is an uncountable mass noun. Second, this is a negative sentence talking about something not existing in a location. When you say "there is no [X]" or ask "isn't there any [X]?", the thing that is missing must always be in the partitive case.

Why is the verb ole instead of on?