Learn French: Voici le détecteur qui trouve les fuites de gaz. - Here is the detector that finds gas leaks.

When a second noun acts like an adjective to describe the first noun (creating a compound noun like 'gas leak'), you generally just use de without a definite article (le/la/les). Using du gaz (de + le) would mean 'leaks of the specific gas' rather than the general category of 'gas leaks'.

How would I use 'gas leak' with the phrase en cas de from our current lesson?

You would say en cas de fuite de gaz (in case of a gas leak). After en cas de, we typically drop the article (un/une/le/la) when referring to a general event or emergency. Notice that it is also usually singular in this context.

Quick refresher: why is it qui and not que in this sentence?

We use qui because it acts as the subject of the verb trouve (the detector is doing the finding). You would use que if the relative pronoun were the object of the verb, for example: Voici le détecteur que j'ai acheté (Here is the detector that I bought).

Can I use Voilà instead of Voici here?

Yes! While voici technically means 'here is' (pointing to something close) and voilà means 'there is' (pointing to something further away), modern spoken French frequently uses voilà for both. Using voici simply adds a slight emphasis that the detector is right here in your hands or immediate vicinity.