In Q. Memento Mori--Revisited offered gerundive (neuter impersonal) alternatives to "memento mori": "nunc est moriendum" = "now must die" & "mox erit moriendum" = "soon one must die". Of course, yourself will recognise these as having been inspired by your own Q:Nunc est bibendum: gerund or gerundive?.
The gerundive genitive is a more complex species: "memento moriendi" cannot be translated as "remember one must of dying". The more long-winded gerundive-of-obligation style:
"Remember it-ought-to-be-(a time)-of-dying/ of-death ("a time" being understood); alternatively: "Remember it-ought-to-be-(a concept)-of-dying/ of-death (again, "a concept" is undertood). This, second one is reminiscent of what Joonas has already said.
These may appeal to literal-translation purists (like me); but, they are clumsy, jerky & rambling. A more succinct offer may be: "miles Romanus, memento mox erit moriendum" = "Roman soldier (rank is now irrelevant) remember soon one must die."
The Romans were well-aware of the rapid passage of time (tempus fugit). This continues to fit with the teachings of Seneca--everyone is dying--a Roman mindset.
The next one: "me paenitet vivendi" = "it makes me sorry of living", it's not a good translation; but, the grammatical rule for impersonal verb paenitet with construction (+ accusative of the person and genitive of the cause) allows for genitive of the cause of the sorrow, here, living.