A brief look in via the google-books, finds that "imperative" (meaning a requirement) appears in two locutions. The first is "X is imperative"+verb form. This is your example. From Physical Abusers and Sexual Offenders: Forensic and Clinical Strategies by S. A. Johnson
... I believe that reading the following authors' works is
imperative to understanding the sexual offender....
The "to" here means "for [the purpose of]", and it would be just as acceptable to say "for understanding." However, the infinitive: "reading is imperative to understand" is much more common.
The second locution is "the imperative is"+verb form. From Becoming Canonical in American Poetry by T. Morris:
If in imagining the future we are constrained by our past, one
imperative is to understand that past ....
In this case when the complement is a verb form, it's the infinitive. I was unable to find a gerund to take that role.