I don't know if I would complain as such OP, but as this is an out of hours activity could you turn up a bit early to collect him from the next one and perhaps use your judgement to decide if you want to have a word with the coach in question after seeing him in action.
If this was your GS's first time at the club, was it also the student's first time coaching it? Perhaps his actions were misguided because of first time nerves and a habit of speaking like that during games at the college with fellow students. It could have been a one off slip, compounded by his explanation of what he meant by it.
Probably the best thing to do, if you decided to do anything, is explain to your grandson why it's not a word you would use or a fair description of girls and although it may be something he hears used by other people both at sporting events and elsewhere, it's not something most people would necessarily say or believe. Point out that there are a lot of accomplished women involved in sports, including those that are traditionally "men's sports" and then just keep listening to his talk about his lessons to see if he mentions any other comments like this from the coach.
I am a little surprised at how many people are defending the coach using the words "bitch out" and ignoring his definition of it ("apparently the coach said that it means acting like a girl when faced with bigger players etc.") to suggest it is just a harmless unisex word implying that the boys were complaining a bit or acting scared of the other players.
Especially since we have a thread going somewhere about sexist christmas presents from the PTA, which seemed to have everybody up in arms about the potential damage to the girls who were to receive them. How can we infer sexism from a Christmas present but be perfectly happy about words like "bitch out" and definitions of it meaning to "act like a girl"?
There is a difference between "Don't tell so-and-so I did that", "Let's keep this a secret" and "We are making this as a surprise."
Although my son is still a little too young to understand all this we are trying to teach him the difference between good secrets ("don't tell Grandma what we bought her for her birthday, it's a surprise...") and bad ones ("Your Mum and Dad will be cross with us both if you tell them, it's our secret...") and promise him that we will never be angry with him for telling us the truth.
If this is a one off slip of the tongue then it's probably not the end of the world that he asked the boys not to tell the teachers. After all, your GS still told you so he didn't ask them to keep it a secret from everyone. As long as your GS knows he can tell you anything, even if he is asked not to, then I think you are okay on that one.