The following scriptures are the only ones that refer to these Hymenaeus and Philetus.
1 Timothy 1:20
of whom is Hymenæus and Alexander; whom I have delivered unto Satan, that they may learn not to blaspheme.
2 Timothy 2:17
And their word will eat as doth a canker: of whom is Hymenæus and Philetus;
Their teachings were blasphemous and ate as doth a canker (strongs: gangrene/mortification) which signifies the
"terrible and deathly nature of the “word” of these false teachers." (Ellicott on 2 Ti 2:17)
We can conclude that they taught extremely contrary doctrine to the word of God but cannot ascertain that they were Gnostics.
Were they Gnostics? Maybe.
But I'd lean on the side of no in my personal opinion.
They probably were only teaching things such as that in the following verse of 2 Timothy rather than mystical esoteric type stuff. Taking the doctrines & commandments of God and flipping & twisting them into a lie to subvert those weak in faith from hope, as Satan and his antichristian followers do.
2 Timothy 2:18
who concerning the truth have erred, saying that the resurrection is past already; and overthrow the faith of some