What is the most important skill for Requirements Gathering ?

The ability to listen & then ask questions that draw explicit details.

Ensure that your truely gather requirements rather than solutions

In addition to the above answer i would like to say specific analytical skills is needed in distinguiging the busniess requirement and the user requirement (points of view) so as to make use of both in appropriate places.

Listening skills !

Relationship Building

• Your ability to convince others to work together towards a common goal (agreeing on requirements or processes) is invaluable.
• You act as both a guide and specialist on the subject of requirements gathering, you engage others in the process without taking over.
• You convince others to work with you as a team, continuing to deliver and receive current, actionable feedback to support continuous growth of the project, while ensuring that the project requirements remain in-scope. (Review out-of-scope needs later).
• You are not only a problem solver but also have the ability to influence others to share in the problem solving and therefore take an active stake in the outcome.

Communication

• You actively listen to the stakeholders to understand their perspective and ensure continuous and complete understanding regardless of communication method or audience.
• You realize that the stakeholder’s terminology is different from what an IT department uses so a first step may be to make sure you understand the vocabulary the stakeholder is using. (You realize it is your job to understand them, not the other way around.)
• You know to translate for the developers and then translate for the stakeholders the information from the developers, so that the information can flow both ways. (Many times this is where the process flows will really come in handy)

Communication

Looking to grasp details of the requirements.

Your logic has to be driven by what my client wants in  general then in details then confirm the details by different type of quastion if possible , at least for those kernal details. This skill called customer requirements drilling