What sort of deferred gain?

It was rental property that was sold in a prior tax year (client is unsure what tax year specifically).  I did not prepare that tax return nor do I have a copy.  I did forget to mention that this is for a LLC (Form 1065) where there are 20+ rental properties.  

You need a copy of that prior year return so you know what was done and what still needs to be done.

Before you know what to report, how to report, and where to report, you need to find out which code section applies, how that arose, what was reported previously, what triggered the recognition, etc.  Time to ask more questions of the client and obtain copies of those prior year returns.