Ross Hind was arrested last May when officers were called to the complex in Lennoxtown, East Dunbartonshire, following reports of a weapon being seen.
The 20 year-old, of Milton of Campsie, admitted a charge of having an imitation firearm without lawful authority when he appeared at Glasgow Sheriff Court.
His own lawyer branded the incident "stupid" and said Hind had lost his job in the army as a result.
He was handed a community payback order by Sheriff Richard Clark, with the condition he will carry out 100 hours of community service, after being told he "recognised the stupidity".
The sheriff told him that while he had knowledge of weapons, others might not know it was not real.
He added: "I have noted as a result of your actions you have lost the opportunity to pursue your career."
The alert came around the same time Celtic boss Neil Lennon had received parcel bombs.
A total of seven people – including Hind – were initially arrested at time.
On the last occasion when he pled guilty he was joined in the dock by Gary Booth, 18, and Katrina Henderson, 22, who faced the same allegation. They walked free after prosecutors accepted their not guilty pleas.
Hind's lawyer Gerry McClaysaid: "This [incident] is basically stupidity rather than anything else."