Neither library bundles or depends on Graphviz directly, you need to install Graphviz yourself. I'm wondering what kind of issues you're afraid of? Are you banned from using software that is licensed under a copyleft license somehow?
You’re correct. I think it’s ruby-graphviz that is (was?) under GPL.
Either way, as a lawyer, I wouldn’t let anything copyleft touch a project that is intended to be proprietary. If nothing else, the drafting of copyleft licenses is generally abominable, and prone to unintended consequences. For non-commercial projects, they’re fine but of marginal utility at best.
