You asked two questions. Your second question, how do you say the art of being loved, is easier to answer. As always, there are many ways to say it. Off the top of my head, I'd start with Ars ejus, quod est amari (the art of what it is to be loved).
Your first question, what is the genitive of a passive gerund, is much more interesting and much less straight forward. The reason is that there is no such thing as a passive gerund, so far as I know.
But passive infinitives are not the only infinitives which lack a gerund. Posse and esse also lack a gerund, and that brings to my mind the neo-Latin expression, A posse ad esse (from being able to merely being)
Here we have two infinitives serving as the objects of prepositions, which is normally a no-no. You should use gerunds as the object of your prepositions. But since posse and esse have no gerunds, Liebnitz and Kant (and a host of others) use the bare infinitives as neuter indeclinable nouns (which is exactly what the grammar books say they are).
But using an infinitive as the object of a preposition is not entirely a neo-Latin novelty. Woodcock (#27) cites a few rare instances of Plautus, Cicero, and Valerius Maximus using infinitives in less usual ways. As an object of a preposition he cites Cicero (Sen. de Ben. 5, 10) inter dare et recipere (between giving and receiving) and Horace (Sat. 2, 5, 65) praeter plorare (beyond weeping).
So if you are not entirely averse to using a mostly unattested form, you might want to consider going the preposition route, something like ars ad amari (an art for being loved), or ars de amari (an art concerning being loved).