Doreen Valiente had to argue with him when he tried to get her to step down as High Priestess because she was "too old".
Also, according to Lois Bourne, Gardner was deeply homophobic (though this is perhaps surprising given his interest in alternative practices such as naturism).
Of course, we have to remember that he was a man of his time, and make allowances for some of his attitudes. He also had many fine qualities - wit, charm, originality, etc. And there are several things I am eternally grateful to him for:
- he did not insist on being reverenced as Our Glorious Founder, and would make adjustments when people argued with him.
- he gave out different Books of Shadows to different High Priestesses, so there could not be a canonical version of the BoS (no matter how much some people want to turn it into the Wiccan equivalent of the Bible).
- he encouraged his covenors to use the bits of ritual they liked and not bother with the rest
If we compare him to other founders of religions, both ancient and modern, we can see that their religions either declined after their deaths or became rigidly dogmatic. So it is much better to have a flawed and human founder whose opinions you can question, than one whose every word is taken as a divine utterance. So he was great - and Wicca is a flexible and adaptable tradition - precisely because he was flawed.