In a word: tangy. It was dark red and had the consistency of sweet-and-sour sauce. It tasted a bit like sweet-and-sour sauce too but with a mango or tamarind flavor.
The waitress tried to steer me to one of their other, more mainstream sauces. She didn’t know what gland is in it. In fact she wouldn’t confirm whether it contains any monkey parts at all. Apparently they buy it rather than make it. She said maybe from South Africa, but the whole conversation was making her obviously uncomfortable at that point so I think she was just saying whatever she thought would get her out of there the quickest.
Clearly more investigation is needed. I’ll try googling it later.