Gallimard doesn't suspect that Song is a man. In court, he even testifies that he didn't know. Ultimately, it appears he doesn't want to believe that Song is a man, which would ruin the fantasy in which he is living.

Gallimard is so convinced that Song is a woman that he provides support for her supposed child that he believes he fathered. This is despite never seeing her nude. She claims that she's too modest to appear nude in front of him. When she returns with their child and Gallimard leaves his wife, they stay together for twenty years.

After the deception is revealed, Gallimard says, "I’m a man who loved a woman created by a man. Everything else simply falls short." He wants to live within the fantasy forever, and only Song revealing his real gender in court disrupts that fantasy. Even if Gallimard had suspicions, he would have repressed and ignored them to the point where it wouldn't have registered with him.

In M. Butterfly, Gallimard never suspects that Song is a man until Song's true gender is revealed to him, but even then, as the previous answers say, Gallimard does not want to believe that Song is a man. This is because the realization that he has been in love with a man for almost twenty years is devastating. Song was Gallimard's perfect idealization of a woman—but this was just an act based on lies. Song's testimony in court is a betrayal, and it humiliates Gallimard. After seeing the truth, literally, Gallimard still chooses fantasy over truth. He says, "I am a man who loved a woman created by a man." He loves and chooses Butterfly; he does not love Song. Song merely created Butterfly for him to love. Gallimard's sacrifice/suicide at the end of M. Butterfly is a way for him to solidify the connection between him and Butterfly from Puccini's opera. He has become Butterfly—the one who has been manipulated.

Gallimard never once suspects that Song is a man. Throughout the course of the play, we see Gallimard's infatuation with the stereotype of an "Oriental woman" as being submissive, beautiful, and perfect. He is so blinded by his belief in this stereotype that Song is able to act the part and never once raise suspicion, despite the relationship being intimate.

Even in court, Gallimard loudly proclaims his ignorance that Song is a man, much to the disbelief of everyone present. His fantasy is so entrenched in his mind that he cannot accept the idea that he has been lied to for years. He continues to deny and chooses to believe that he is being lied to now rather than accept the fact that Song is a man.

Not only does Gallimard never suspect that Song is man, but he does not even want to believe the truth after Song's true identity is revealed. No one of course believes this because it seems absurd to carry on an intimate, long term relationship without knowing the sex of one's partner. However, Gallimard maintains that he did not suspect that Song was a man. Gallimard is totally wrapped up in his fantasy of the perfect woman, and this fantasy pervades his relationship with Song. Keep in mind also that Song goes to great lengths to conceal his true identity from Gallimard, and Song uses Gallimard's desire to believe in the perfect woman to his advantage in the concealment. I do not think that Gallimard is gay; he loves Song as the ideal woman because she embodies all the stereotypes that suggest he is superior in his masculinity. His being gay would not support this. Rather, as Song says, the perfect woman could only be created by a man, so Song creates Butterfly to appeal to Gallimard's weaknesses.