True enough.

I had one as a teenage mod, a co-worker talked me into buying it from him. Grey, fairly oppressive and permanently damp. It soaked up water like a sponge, so no, not really resistant- except perhaps in the lightest of showers. I wore mine for probably a year until I replaced it with a full length sheepskin from Kensington Market shortly before it closed.

Gannex worn by former PM Harold Wilson

Gannex coats were issued when I joined the police in the 70s, a lot of forces had them. You choice of outerwear was a double breasted gaberdine coat with a button-in liner or the Gannex which was generally preferred as it was fly fronted and presented a cleaner look, it was also genuinely waterproof. You also got a cape which was very warm, but no good if you had to fight anyone, also kids shouted ‘Batman’ at you from a safe distance.

Gannex macs probably wouldn’t make it as an Ivy garment, Harold Wilson didn’t have the charisma of President Kennedy.