"An examination was made of the hostile act committed against the Korean nation and the trespassing on the border of [North Korea] against which an indictment was brought in and his guilt was confirmed," the agency said.
It added that Swedish diplomats in Pyongyang, who represent US interests in the North, had witnessed the trial.
North Korea said last month it had arrested Gomes, 30, of Boston on January 25 for trespassing after he crossed into the country from China.
"Needless to say, they are disturbed [by the sentence] but they are hopeful that he would be returned home to them and they are praying for that"
Thaleia Schlesinger, Gomes family spokeswoman
Thaleia Schlesinger, a spokeswoman for Gomes' relatives, said they are "disturbed" by the verdict and will pray for his early return.
His family were "praying for him and hoping for his return home as soon as possible," she said.
"Needless to say, they are disturbed [by the sentence] but they are hopeful that he would be returned home to them and they are praying for that."
Yoo Ho-yeol, a North Korea expert at Seoul's Korea University, said he believed Gomes will eventually be released as the North appears to want to use his case as a bargaining chip in negotiations with the US on its nuclear programme.
"Continuing to hold him in custody is also a burden for North Korea" as it will only galvanise criticism of its human rights record, said Yoo.
North Korea is under international pressure to return to stalled six-nation nuclear disarmament talks it quit last year.
Gomes is the fourth US citizen to be detained on charges of illegal entry into the North over the past 13 months.
Last August North Korea freed two US journalists who had been sentenced to 12 years hard labour following a surprise mercy visit by Bill Clinton, the former US president.
And in February this year the North freed a US missionary, Robert Park, who had been arrested two months earlier after allegedly crossing the border with China.