While you can designate anyone to receive the proceeds of your Federal Employees' Group Life Insurance or Thrift Savings Plan account, the rules are much stricter about who is eligible to receive a survivor annuity.
Only a lawfully wedded spouse or former spouse (when there is a court order) is entitled to a survivor annuity. As a rule, a nonspouse — companion, significant other, life partner, etc. — of a federal employee or retiree isn't entitled to a survivor annuity. However, there is an exception: The Office of Personnel Management will recognize common-law marriages for survivor benefit purposes, but only if the state in which you live either recognizes it, or if that common-law marriage was contracted in a state which does recognize common-law marriage and you now live in a state that doesn't.
There also is a little known provision of law that allows you to provide a survivor annuity to someone who has "an insurable interest" in you.
Individuals who are presumed to have an insurable interest in you are your current spouse (if blocked by a court order), a blood or adoptive relative closer than a first cousin, a former spouse, someone to whom you are engaged to be married, or someone with whom you would be considered to be in a common-law marriage in a place that recognizes such arrangements.