Gabbard, who served in the Iraq war with the US army, said she had not planned to meet the Syrian leader, but that she was "given the opportunity" and decided that it was "important to take it".
The congresswoman also urged the new US government to stop providing support to the factions opposed to al-Assad, which she described as "terrorists".
"The US must stop supporting terrorists who are destroying Syria and her people. The US and other countries fueling this war must stop immediately. We must allow the Syrian people to try to recover from this terrible war," Gabbard said in her statement.
Gabbard told local media that the main groups fighting in Syria against al-Assad are the Islamic State and the al-Nusra Front - a former al-Qaeda affiliate in Syria - and said that other factions are fighting with them or under their command.
"There is no difference between 'moderate' rebels and al-Qaeda or the IS - they are all the same," Gabbard added.