Was Gerald Gardner homophobic?
— Very probably, yes. (See Lois Bourne's memoirs.)
Does this matter for practitioners of modern Wicca?
— Because whilst Gardner is respected as the founder of modern Wicca, his views on many issues are seen as a product of his time. in the 1950s and 60s, not being homophobic was the exception, not the rule. I can remember as recently as 25 years ago, it was rare to find a heterosexual man who was not homophobic. Now, thankfully, it is regarded as abnormal to be homophobic (at least in the circles that I move in).
Gardner does not have the same status in Wicca as Jesus does in Christianity. He is not believed to be some kind of messiah figure. Wiccan practice and tradition is not about Gardner, it's about magic and Nature.
There are some practices in Wicca that are heterocentric, but people are changing these to be more inclusive of LGBT people. If the coven you are in is not changing to include you, there are other covens that might be more flexible. There's also the option to start your own coven.
These heterocentric practices may have been started because of Gardner's homophobia, so in that sense it is still relevant, but for heavens' sake, Wicca is less than a century old, so it's perfectly possible to change things. Tradition is not set in stone; it evolves.
A good place to start is the excellent book, Cassell's Encyclopaedia of queer myth, symbol and spirit.
Further reading on LGBT sexuality and Paganism.