I'm planning to use
mosaico (https://github.com/voidlabs/mosaico/blob/master/LICENSE) in a closed source commercial web application. I wouldn't make any modification to the library itself. Most of the GPL questions and FAQs talk about static/dynamic linking, binary libraries etc., which are not trivially applicable for a web framework use.
I don't think I'm technically redistributing this library, but I'm uncertain about this. The web application would function even without
mosaico would provide a nicer email designer capability than our existing one.
I cannot make our software open source, however the web front-end of our application is kinda accessible (can be crawled), but we don't have a public facing source control repository.