All three pairs can be conceptualized as things which go together to form a single unit, so it is not unusual for people to use "is" as the verb. For the first two, you may use "are" if you wish, but "gin and tonic" consists of the two liquids mixed together so it's nearly impossible to think of this as a plural. Unfortunately for learners from other cultures, you need to know which things are considered combinations and which are not -- from the viewpoint of the culture of English-speaking people.
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