Gender-neutral version of widow & widower?

One good option is surviving spouse:

A spouse who lives longer than the other spouse.
_{US Legal}

This expression is pretty widespread and is used on the Social Security webpage:

Benefits can be paid to the worker's children and the surviving spouse who is caring for the children even if the worker doesn't have the required number of credits.

It's also used on the Office of Personnel Management Retire FAQ (another government site):

If you are the surviving spouse of a deceased employee, recurring monthly payments may be made to you if your spouse completed at least 18 months of creditable civilian service and was covered under the Civil Service Retirement System (CSRS).