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Marler apologised to Lee, who is from a Traveller background, at half-time during England's Six Nations win.
Traveller campaigner Shay Clipson said Marler should be banned.
"It's not just in sport, I think it's in every aspect of life where people get so PC and just make massive issues about things," said coach Gatland.
"It was just one of those things, a little bit of banter. We've accepted that and we've accepted his apology. Just move on.
"Fifteen or 20 years ago, those things were sorted out with fists and stuff."
The comment by Marler, 25, is under investigation by Six Nations officials.
Clipson, the founder of the National Alliance of Gypsy, Traveller and Roma Women, told BBC Radio 5 live that the Harlequins prop "cannot make mistakes like that and expect to just say sorry and walk away".
She added that the public had to imagine what the reaction would have been had a black or Asian player been subject to that sort of derogatory remark.
Marler has been cited for a separate incident of foul play involving another Wales prop, Rob Evans.
A previous Welsh Rugby Union statement said: "We're disappointed by the comment made and pleased Joe Marler apologised."
On Monday, assistant coach Rob Howley commented: "As players and coaches, there's banter in the game, but I think there's no place for that."
Marler's comment was heard by television viewers after being picked up on referee Craig Joubert's microphone.
"In modern sport, players have got to be aware that there are microphones everywhere and you have to act accordingly," said Gatland.

Most of the passengers are reported to be from Latin America.
The crash happened on Interstate 10 close to the resort town of Palm Springs in the early hours of Sunday morning.
Investigators are examining the cause of the crash, which happened as the bus was bound for Los Angeles.
Rescuers used ladders to climb into the bus windows to remove bodies. Five of the injured passengers are in critical condition.
The Los Angeles Times described the crash as the deadliest in California for several decades. It said that the bus careered into the rear of the truck at high speed, mangling the front third of the bus.
Most of those who died were apparently at the front of the bus, the newspaper reported. The driver was among the fatalities.
Officials quoted by the paper said that identifying the victims could take days, because some were not carrying identity cards or had lost their belongings when they were taken to hospital.
They have warned that it may not be possible to determine exactly why the accident happened because of the driver's death.
Investigators will assess whether he might have fallen asleep or had a heart attack, in addition to determining if there was a mechanical failure or some other kind of emergency.
The westbound lanes of the interstate were closed near the crash site but are now reported to have been reopened.
According to the Desert Sun newspaper, the tour bus, run by US Holiday, was coming from Red Earth Casino, near Salton City in California.
The small Los Angeles-based company, runs trips to casinos in California and Las Vegas. The driver of the bus was one of the owners of the tour company.
The Federal Motor Carrier Safety Administration records show the company had one vehicle and one driver. According to the data the company held a satisfactory safety rating and had not been in a crash for two years.
Warren Gatland says the "Gypsy boy" comment aimed at Samson Lee by Joe Marler was "banter" and insists Wales have no issue with the England prop.
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Thirteen people have been killed and more than 30 injured in southern California after a tour bus ran into the back of a lorry, media reports say.