Name of an old book? Edgar Rice Burroughs style and time?

I used to have an old book but I can't remember what it was called. It was in a similar style as Edgar Rice Burroughs but had some mature tones. It was written around the same time as ERB's books, I think. I don't think it was ERB, because I can't seem to find anything of his that sparks a memory with it. It had a character in it named Gunther that was, if I remember correctly, the main antagonist. Kinda had a The Lost Continent/Beyond Thirty by ERB feel to it. And I may be wrong but I'm pretty sure it wasn't the same. Any suggestions of something that sounds familiar or similar for a new read even is very much appreciated.