I ask this question because I don't know if we can enchant a guilgate, like a boros guildgate as example, with Chained to the Rocks.
No it is not. Mountain is a subtype of a land cards; for Boros Guildgate, the subtype is "Gate".
However, Steam Vents is a mountain and can be enchanted with Chained to the Rocks, because it has subtypes "Island" and "Mountain".
"Mountain" can mean two things:
When a card refers to something that can be a name or a type, it always refers to the type. As such, Chained to the Rocks looks for objects with the land type "Mountain".