Gambari noted that Aung San Suu Kyi, the detained opposition leader, was allowed to issue a statement for the first time in more than four years and to meet members of her National League for Democracy party.
And he said although his visit had not produced all the results he had hoped for, he would return to Myanmar "again and again and again" to promote "substantive dialogue".
On the same day he gave his positive report, however, activist Su Su Nway was arrested in Yangon, Myanmar's biggest city, after being on the run since the army crushed protests in September, an opposition source said.
Gambari's report on Myanmar was met by scepticism from UN ambassadors who said the military government had not shown any firm commitment towards democracy.
"Until the generals' military hardware is crumbled, they won't listen to anyone"
Oomlwin, Yangon, Myanmar
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John Sawers, Britain's ambassador to the UN, warned that signs of progress, if not backed up by sustained international pressure, "could also be a false dawn".
Zalmay Khalilzad, the US ambassador, said the positive steps did "not yet constitute a fundamental shift" unless the government showed its commitment by releasing Aung San Suu Kyi and other political detainees.
He said the fact that the military government had arranged the schedule for Gambari's visit was "troubling" and "raised questions about their sincerity".
"The regime's so-called road map to democracy is demonstrably inadequate," he added.
Khalilzad said it was important to balance engagement with pressure, adding: "We do not rule out sanctions".
Responding to the scepticism, Kyaw Tint Swe, Myanmar's ambassador, said his government was co-operating fully with the UN and making progress.
"It's disappointing that, notwithstanding the positive developments, some continue to express scepticism."