Method of finding gaps in time series data in MySQL?

Lets say we have a database table with two columns, entry_time and value. entry_time is timestamp while value can be any other datatype. The records are relatively consistent, entered in roughly x minute intervals. For many x's of time, however, an entry may not be made, thus producing a 'gap' in the data.

In terms of efficiency, what is the best way to go about finding these gaps of at least time Y (both new and old) with a query?