How to write a spike when using BDD? If Gherkin is used to write regular development user stories in the Scrum process, should Gherkin be used to write a spike as well? If so, given that often there is a lot of unknown information to be filled in, how can a spike be written as BDD?

Generally speaking, no.

More to the point, you should not expect a spike to be described in Gherkin. The primary purpose of BDD is to focus coding efforts toward a clear goal. When working a spike, your goal is a sort of validated exploration of a problem, so the BDD approach is overly limiting to the point that it works against the purpose of the spike.