Gandharvas were a different tribe of creation just like devas, apsaras or rakshasas.
According to the Vishnu Purāna, they were sons of Brahmā.
"The Gandharvas were next born, imbibing melody; drinking of the
goddess of speech, they were born, and thence their appellation. (gām
dhayantah, 'drinking speech')."
In another place in the same Purana, it is also mentioned that they were the offspring of Kasyapa and Arishtā, and therefore grandchildren of Brahmā. The Padma Purāna speaks of them as the children of Vach.
In Hindu mythology, the Gandharva was a minor deity in the Rig-Veda, however in later writings they were described as a class of semi divine beings.
In Fact, Vishvavasu is probably the oldest Gandharva mentioned who is also the composer of Rigveda ( 10.139) by the name Vishvavasu Devgandharva
The following is the Ananda Valli from the Taittiriya Upanishad explaining the different levels of happiness among various species of life.
te ye satam manusa anandah | sa eko manusyagandharvanamanandah |
srotriyasya cakamahatasya |
"One hundred such units of human joy make up a single unit of joy
which the manushya gandharva possesses. A sage full of revelation and
free from all cravings also possesses the same joy."
[Gandharva in general refers to a species of heavenly musicians. There
are two categories of gandharvas mentioned in these verses. The first
is the manushya gandharvas - or human gandharvas, and the second is
the deva gandharvas - or heavenly gandharvas. On the earthly planet
there are many realms of existence that are inaccessible to the
ordinary humans. These realms exist on high dimensions of space.
According to the Vedic conception of reality there are 64 dimensions
of existence, of which ordinary humans interact with three. The
manushya gandharvas live in higher dimensions within the earthly realm
of existence. There are many descriptions within the Puranas of lands
existing on this planet that are inaccessible to us. For example,
there are three different levels of the Himalayas. What we experience
as the Himalayas is only the lowest dimensional level. It is described
in the Mahabharata that Bhima was able to visit many of these higher
dimensional realms when he was collecting wealth for the rajasuya
yajna. Also the palace of Yudhishthira Maharaja was created by
yakshas, who exist on higher dimensions. The artwork they made for the
palace (paintings, carpets, ceiling, etc.) was actually animated and
in three dimensions. So much so that it was not possible to
distinguish it from reality. For those who are existing in three
dimensions, a painting will be done in two dimensions. But for those
existing in four dimensions, a painting will be done in three
dimensions. This is why Duryodhana was confused when he entered
Yudhishthira's palace, and ended up stepping in a pond, thinking it
was the floor.]
te ye satam manusya-gandharvanamanandah | sa eko
devagandharva-namanandah | srotriyasya cakamahatasya |
"One hundred such units of joy which the manushya gandharva possesses
make the joy of the deva gandharva. A sage full of revelation and free
from all cravings also possesses the same joy."
[This verse is describing the heavenly gandharvas. They are a heavenly
species. Their leader is Vishvavasu, who guards the soma rasa. This
category of gandharva mainly makes up the celestial musicians. They
entertain the higher devas and perform in Indra's court.]
te ye satam devagandarvanamanandah | sa ekah pitrinam
ciralokalokanamanandah | srotriyasya cakamahatasya |
"One hundred such units of joy which the deva gandharva possess make
the joy of the Pitrus who inhabit the long enduring would. A sage full
of revelation and free from all cravings also possesses the same joy."