In English, I have seen many books and videos with titles like:
Understanding Quantum Mechanics
Finding Nemo
Going away
But how do I say something like the above in German?
Notice that there is a different cultural background at work. Some things are simply done (and said) differently in English and German. Here are some suggestions but keep in mind that a title needs to be interesting. Regurgitate a formula which has been used over and over and you get the exact opposite of that.
Use the topic itself, e.g. "Quantenphysik". That the book teaches how to understand quantum physics and is not written with the intent to maximise confusion of the reader is something of a given, no? It is possible to have a subtitle, like "Quantenphysik - einfach erklärt" (explained simply) to set a book for laymen apart from, say, a book for physics students.
Titles which make use of the present progressive like in English are not common at all in German. Furthermore English speakers commonly use the result of an action whereas German speakers will use the action itself: "Finding Nemo" would rather be "Die Suche nach Nemo", (the search for) because the book is not about finding the fish but rather the search for it.
If you want to title something the yellow-press would suggest to its readers you can add a "leichtgemacht" (about: "the easy way") to the topic. "Going Away" would become "Weggehen - leichtgemacht". Notice that this type of titles is so abundant that you shouldn't use it if you have something relevant to say. "Quantenphysik - leichtgemacht" will perhaps sell not a single copy because people interested in quantum physics will be put off by the "leichtgemacht" and people who appreciate "whatever - leichtgemacht" will not be interested in quantum physics.
German book and movie titles aren't like that. As simple as that.
A book titled “Understanding Quantum Mechanics” would be e.g. titled „Einführung in die Quantenmechanik“ in German. Or „Grundlagen der Quantenmechanik“, „Quantenmechanik für Ingenieure”, „Lehrbuch der Quantenmechanik“ or if the authors love puns „Quantenmechanik in kleinen Schritten” or something like that.
And similarly for “Finding Nemo”. That one was named “Findet Nemo” in German with the imperative.
That “Going away” movie is originally French and likely the German publisher would translate the French title instead and call it „Ein herrlicher Sonntag“ or similar.
I understand your question to be about how to translate the English gerund to German, rather than about how movie titles are translated. In German, the infinite is used. The literal translations of your examples would be
Quantenmechanik verstehen
Nemo finden
Weggehen