Gilchrist said that unless Harris had a genuine niggle, he should play, regardless of the fact the series is over.
As Australia's great success story for the series, along with opener Chris Rogers, Harris deserves a chance to finish strong and lay a marker for the coming series, according to Gilchrist.
"While mindful of the opportunity to rest guys who may be injured and therefore miss future matches, I think Ryan himself deep down would dearly like to play at The Oval," Gilchrist wrote on EspnCricinfo.
"When you've missed as much cricket as he has through injury, and don't know how much you have left, you want to take every opportunity to go out and play.
"Australia have a chance to show some of the resolve they demonstrated for most of the two Test matches at Old Trafford and Chester-le-Street, and try to get the kind of result that would lead to a more positive mindset when they start the return series back home.
"I've always felt that if they're all fit, you pick your best XI, and under those terms Ryan is just about the first man who should be chosen."
Meanwhile, England star James Anderson is desperate to send Australia a message by playing in the fifth Test at The Oval.
Even though the match is a dead-rubber and Anderson seems in need of a rest, the strike bowler feels he has unfinished business in this series and a point to prove ahead of the return Ashes in the summer.
Anderson took 10 wickets in a match-winning performance in the first Test at Trent Bridge.
But since then he's struggled to back that effort up and in the fourth Test at Chester-le-Street he was particularly toothless.
There are calls for Anderson to be wrapped in cotton wool with so much more Ashes cricket still to come, but Anderson has spoken out pleading to be retained for The Oval.
"I desperately want to play in the fifth Test, not only to finish what we started but also to try to rediscover the form that I admit has not been as I would have liked in the last couple of matches," Anderson wrote in the Daily Mail.
"I know it is pretty impossible to perform at 100 per cent in every match, but I'm my own harshest critic and, by the standards I set myself, I did not bowl as well as I can at Old Trafford or the Riverside.
"That makes me want to play this week to try to find out what has been missing and work out how I can get it back.
"Physically, there is nothing to suggest I need a rest. I am nowhere near the state of exhaustion that hit me when I was left out of the side in the last World Cup in India and Sri Lanka, when, at the end of a winter which included our brilliant but draining 2010-11 Ashes tour, I more or less hit the wall."