Most law schools' application windows close in February-March of the year that you plan to begin law school (i.e. if you want to start at the end of August 2018, your application has to be in by February-March of 2018--each school will have exact dates as deadlines but they generally fall in those months).
That means you have to take the LSAT at least a month or so before you apply. Many people take the LSAT earlier, like in October. Unlike the SAT, you can't take the LSAT multiple times and just use the highest score in your application; law schools will look at all your scores. So you have to make sure you ace the LSAT, which means you must spend months beforehand doing practice tests and, if you have the money, a prep course.
If you want to take a gap year, in other words work or travel or whatever during the year after undergrad and before law school, then all this--your LSAT prep, taking the LSAT, and submitting your applications to law school--would happen during your gap year, not during your senior year of undergrad.