Gibson's introduction was one of three Republic changes at the interval with Shane Long and Andy Keogh also coming on.
BBC: SPORT | Football | Internationals | Denmark 0-4 Republic of Ireland
Michael Kightly had earlier missed from eight yards for the home side, while Andy Keogh failed to touch in a cross.
West Brom's Paul McShane and the Wolves duo Darren Potter and Andy Keogh were included in an inexperienced squad last week.
BBC: SPORT | Football | Internationals | Republic squad hit by withdrawals
The prolific striker netted from close-range after good work from Andy Keogh.
Debutant Eddie Johnson fired over for the hosts but City raced away to score the opener on five minutes when Andy Keogh tucked in Steven Caulker's pass.
But the pace of the Bluebirds counter-attack was showcased when Andy Keogh, in for suspended talisman Jay Bothroyd, quickly broke to set-up their first clear sight of goal.
Pompey keeper David James saved an early effort from Matt Jarvis and bravely blocked a late Andy Keogh shot, while Tal Ben-Haim threw himself in front of Kevin Doyle's follow-up.
So Jones is keen to keep the former Arsenal trainee as although Cardiff boast a strike-force of Craig Bellamy, Michael Chopra and Andy Keogh, Bothroyd's role in his side is unique.
Wayne Hennessey, David Edwards and Sam Vokes are off to Denmark with the Welsh squad, while Andy Keogh and Kevin Foley link up with the Republic of Ireland to face Poland in Dublin.
Ramsey is Jones' seventh loan signing this season - joining Bellamy, Koumas, Emmanuel-Thomas, Olofinjana, Danny Drinkwater and Andy Keogh - although the club can only include five loan players in a matchday squad.
The Irish stopper, who is reportedly being tracked by Arsenal, blocked Andy Keogh's close-range header, denied the striker's follow-up after initially parrying an arrowing McIndoe shot and beat away a Jay Bothroyd effort.
Club top scorer Pitman, who had come on as a substitute with Andy Keogh for the hard-working Maynard and Clarkson, settled the issue when he powered home Adomah's cross in the third minute of injury time.
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