Learn Danish: Det er gratis at komme ind.

In Danish, adverbs of place often have two forms: one for movement and one for standing still. Because the action komme (to come) involves movement towards the inside, we use the motion form ind. If you were already inside, you would use inde.

Could I use the word fri instead of gratis to mean free?

No, because they mean different things in Danish. You use gratis when something costs no money (free of charge). You use fri to talk about freedom or liberty, like having time off work or being a free person.

Why do we start the sentence with Det?

Just like in English, Danish sentences generally need a subject. Because the real subject of this sentence is the action itself (at komme ind), we use Det (It) as a placeholder to fill the subject spot at the beginning of the sentence.

How is the word at usually pronounced here?

When at is used as a marker before a verb (like the English "to"), it is almost always pronounced like the Danish letter å (an "aw" or "oh" sound). So at komme sounds like "å komme" in normal daily speech.