The Gandharva clan is one of the eight sura clans. Until the Cataclysm, the clan stronghold was on the planet Carte, but since then it has existed in the sura realm, where the clan's suras are particularly vulnerable to the poisonous environment. In Seasons 1 and 2, the fate of the clan is unknown but is hinted at having been decimated.
As their first king, Gandharva, was among the strongest nastika alive, the Gandharva clan started off with relative superiority in strength. There was a time when Gandharva was in the prime of his strength and was considered cold and cruel, but his wife changed that and influenced him into becoming a pacifist. The Gandharva clan established an alliance with the Garuda clan and the Yaksha clan due to the friendship of the three kings.
However, the King's wife died, which caused Gandharva to have unpredictable mood swings. Moreover, by the year N0, superior suras were sent to the sura realm thanks to Visnu. The toxicity of this realm was dangerous for the Gandharva suras, which lead to Gandharva exhausting nearly all his powers to clear part of the toxic fumes from their area to allow the non-nastikas of his tribe to live. This caused the king to grow weaker than the average nastika.
While Visnu, as a friend of Gandharva, had promised to get another god to make an ocean for his clan to allow them to survive in the sura realm, no one showed up. In fact, when Manasvin pays a visit to Gandharva, he points out that only Varuna would be able to create an ocean under such conditions and considering that she has never appeared, she likely refused. Manasvin proposes a plan to Gandharva to help his clan get an ocean, Gandharva rejects the plan claiming it's crazy.
Eventually the Gandharva clan was attacked, apparently by the Taraka clan, and it is forced to take refuge with the Garuda clan. It was decided that only the strong would go. Shakuntala, the King's daughter, decides to stay behind with the children. Her whereabouts are unknown, but Gandharva is in the human realm aiding Kubera in order to be able to open a portal to the sura realm and find her.
Whether the former Gandharva stronghold in the sura realm is still inhabited by the Gandharva suras, or if it is intact at all, is unknown at this point. Sagara mentioned that the Gandharva clan was annihilated by Taraka clan.
The Gandharva clan is comprised of a variety of sea-dwelling or sea-loving creatures, so most of them prefer living in and around water rather than on land. There are a few members who adapted to living on land too, but such a lifestyle causes them to be considerably weaker. This clan is especially weak against poison, so the poisonous atmosphere of the sura realm was a big problem for them.
The weakness against poison seems to become less if the surroundings contain a large body of water. The suras of the Gandharva clan were desperately asking their King to create a sea for them, however, the best Gandharva could do was to purify the area, which weakened him significantly.
It seems as if Gandharva is a specialist for undercover missions. He has the ability to nearly fully suppress his sura form and features, adapt human looks and behavior, and even pass through highly sophisticated sura detectors. He is even shown to eat vegetables but avoids seafood. It is unknown if these characteristics apply to the rest of his clan.
Because of their respective kings' friendship, the Garuda clan and the Yaksha clan were the strongest allies of the Gandharva clan. However, Garuda fell into a coma and Shuri is "no longer of this world", so the alliance started to decay. Recent events, such as the fact that Maruna's army surrounding Rindhallow is composed only of Gandharva, Yaksha, and Garuda suras, and a flashback of Ran's grandmother talking and aiding Gandharva after N0, seem to hint that the alliance is still working.