Point out how insignificant it is.

Try to put this tiny fix into perspective, and if necessary punch up utube on your phone to show them how easy it is to install one of these things.

It only takes about a minute and a screwdriver.

I agree with Jill Murty, Realtor - Serving Orange County, CA here, and it could also just be an issue of the buyer wanting everyone to do what they're supposed to do, regardless of how small or insignificant it may seem.  Usually a promise, in writing, that this will be completed is enough.

You simply find a way to solve the problem. That is your job. 

Sometimes people use the insignificant issue as an excuse when there's a deeper unresolved problem.  See if you find out if that's the case.

Ask the client if they would agree to sign docs if the GFCI  is installed.  If the answer is yes, ask escrow to contact the realtor (maybe the realtor is still responding to escrow) and ask if he'd like them to draw up an amendment stating the GFCI will be installed at no expense to the buyer prior to the close of escrow. 

Yes, this is silly.  If you can't get a hold of the Realtor, try calling his broker and explain the situation.

If it is small and insignificant, I try to get around it.

I'm a firm believer in doing what you agreed to do. Sometimes it really IS about the principle of the thing. 

Break it down to money - it costs you this many dollars per day to be stubborn and you can replace the GFI for this many dollars. How long will you be stubborn?

Call the broker and or the homeowner.  I've never had such an issue, others yes, but that's just dumb.

Chris Mamone Time to call the broker and get this sorted out.

There is a cumulative effect in Real Estate and this "client" has reached his limit. Let's say it's a $100 to install this GFI outlet, who's fooling who when hundreds of thousands of dollars are potentially at stake Chris?

Someone needs to do their job.  Either have them get it installed or do it yourself.

Jill Murty, Realtor - Serving Orange County, CA 's suggestion is a good route. If the agent doesn't respond to that contact their broker and let them know this agent is not doing serving their client. Be willing to handle the repair yourself with the owner's permission.