Learn Italian: Mi piacciono questi biscotti, tranne quelli con il glutine. - I like these biscuits, except the ones with gluten.

Tranne means "except" or "apart from". You could absolutely replace it with eccetto in this sentence without changing the meaning. Both are used directly before the thing being excluded, without needing any extra linking prepositions.

Why is the verb piacciono plural here?

Remember that with the verb piacere, the thing you like is the grammatical subject. Since you like questi biscotti (a plural noun), the verb must be the plural form piacciono to match it.

Why use quelli instead of questi in the second part of the sentence?

Quelli translates to "the ones" here. While the speaker is referring to "these biscuits" (questi biscotti) right in front of them, they use quelli to refer broadly to any biscuits out there that contain gluten.

Why do we say con il glutine instead of just con glutine?

In Italian, general substances and categories usually require a definite article. So, instead of saying "with gluten" like in English, Italian says "with the gluten" (con il glutine).