Learn Croatian: Možemo spavati u šumi ili gdje god želiš. - We can sleep in the forest or wherever you want.

The particle god adds the meaning of "-ever" or "no matter", turning gdje (where) into gdje god (wherever). Notice that it is written as two separate words in Croatian, unlike the English word "wherever".

Could I use bilo gdje instead of gdje god in this sentence?

Yes, they are very similar, but gdje god is slightly more natural when directly followed by a verb expressing a personal choice, like želiš (you want). While bilo gdje simply means "anywhere" in general, gdje god carries a stronger conditional sense of "wherever that might be".

Why is it u šumi (Locative case) rather than u šumu (Accusative case)?

We use the Locative case here because spavati (to sleep) is a verb indicating a static location. If the sentence used a verb of motion towards a destination, like ići (to go), you would use the Accusative case instead: idemo u šumu (we are going into the forest).

Why is spavati in the infinitive instead of using da plus the present tense?

In standard Croatian, modal verbs like moći (can / to be able to) are followed directly by the infinitive form of the main verb, just like in English. So, we say možemo spavati (we can sleep). You might hear možemo da spavamo in some regional dialects or neighboring languages, but the infinitive is the standard structure for Croatian.