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There are a lot of unknowns in your question.  Are you in a traditional SDLC or Agile environment?  Do you have the time and commitment of stakeholders or future users of the system?  Is it a brand new system or a rebuild of an existing system?

So it's difficult to answer your question.  But in general requirement gathering can be spend up through collaborative meetings such as JAD sessions where all the relevant parties are in a room together and can brainstorm and document requirements together.  However, when the requirements gathering timespan is short there will be misses and oversights.  This makes requirement change management imperative.  You must have a process that allows for changes in requirements and tracks the changes through development and testing to ensure the final set of requirements are met.