In terms of graduate applications, what sort of courses should I take in a gap year?
Linguistics professor here (at a big public university in the U.S.). I think this is an excellent question.

The first thing I want to point out is that all else being equal, that extra year of lab work is going to be an asset when it comes to graduate applications. It attests to you having research experience and lends additional credibility to the areas of interest that you point toward on your statement of purpose. A big extra plus is if the extra year helps to net you any presentations or publications.

That aside, this is an interesting dilemma. On the one hand, this is your last real chance to enroll in classes outside your major; a Ph.D. in linguistics would not typically leave any room for it. Having other interests is still possible in grad school, but a reasonable expectation is that they will be squeezed into a corner (you might read the occasional book about them, for instance). On the other hand, an admissions committee will see your entire transcript, and if you take courses to help you prepare for graduate school, that will not go unnoticed.

My advice is as follows: compile a list of Ph.D. programs that you want to apply to, and then pay very close attention to the preparation they want to see among applicants to their program. Do they strongly advise applicants to have completed a class in linguistic field methods? Do they like seeing applicants with background in (e.g.) phonology or syntax or morphology beyond the introductory courses in those subfields? Is having training in statistics an advantage?

If you've already maxed out on the requirements for the program(s) you apply to, then you have some freedom to select other classes. If you are missing something recommended (or optional but clearly well regarded by the department you're applying to), this is a chance to fill in the gap and become a more competitive applicant. Similarly, one of my colleagues tells gap-year applicants that if they have completed a core linguistics class but scored below about the B-minus level, taking that class again in an attempt to mitigate the weak spot on the transcript is advisable.