Rangers said they were studying CCTV footage in a bid to catch those responsible.
And club bosses said that sectarian behaviour “wouldn’t be tolerated”.
A spokesperson said: “We are studying security camera footage to see if those responsible might be identified.
“Sectarian behaviour, in whichever form it takes, must never be tolerated.”
And anti-bigotry campaigners have slammed the slur – branding it “moronic behaviour”.
Nil by Mouth’s Dave Scott said: “This is pathetic, moronic behaviour which only antagonises people days before a cup final”.
And cops confirmed that they were investigating the graffiti.
A spokesperson for the force said: “We received a report of vandalism to the front gate around 1am this morning.
“Enquiries are ongoing. Anyone with information about this incident should contact police on 101, quoting incident number 0298 of Saturday, 7 December, 2019.”
Rangers have been contacted by the Scottish Sun for comment.
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