Nope, Marin and Gojo don't end up as a couple in the manga. The manga follows a different narrative path for their relationship, concentrating more on other aspects of the story and character growth.
There were many kinds of hand gestures for Gotto Gojo, including greeting, teleportation, and domain expansion. However, the specific description of the gesture was not found in the search results provided. Therefore, based on the information provided, I am unable to accurately answer how to do the gesture of Gojo Guru.
In the manga, the duration of Gojo's seal is not clearly defined. It's left somewhat open-ended, allowing readers to interpret it based on the context and events that follow.