The gate prior to the initial investigation is the first decision point when resources are committed to working on the proposal; it is also the point at which the potential project is first formally recognised. This gate is unique in that it is the only one in the project life cycle which does not require you to have a plan for how you undertake the work which follows.
‘One of the greatest pains to human nature is the pain of a new idea.’
WALTER BAGEHOT, 1826
The proposal describes the business need (i.e. it focuses on why you want a project) and, if known, what you want to do. You should document it formally and have it reviewed by potential stakeholders ...