Well, the gerund would be "the leveling off of" (e.g., "the leveling off of gas prices"; although that might read a bit clumsy in many sentences).
Could you give an example of the sentence you are trying to write (or the full question and responses, if this is a question and answer)? Without more context, I'd say that in most cases you can use the verb instead of the noun to keep your language more direct.