What is Priority in GTD?
It is the last criterion of the Four-Criteria Model for Choosing Actions. Given the context you are in and the time and energy you have, the relative priority of the actions that meet those criteria will determine which one to choose.
This priority is determined by multiple factors (responsibilities, goals, values, etc.) that your intuition has been informed of when reflecting on your different levels of perspective. Trust your intuition to choose what action to take ultimately.