Here’s the pinyin for his name: Gūo Hóngzhì. I’m out of practice and someone will likely come and correct me, but I believe that (ignoring tones) would be pronounced something like Gwa Hong Jer.
That’s about right. I’d say that the best approximate pronounciation would be something like: Gwoh Hoong-jer. Typically, in the Taiwanese transliteration system (which I think is Wade-Giles), the “k” in “kuo” would be pronounced as “guo” while the “k” in “k’uo” (with the apostrophe) is pronounced as a regular “k” sound. Unfortunately, most people don’t use the apostrophes when they transliterate their names, so it’s difficult to tell.