Creep in is a common phrase used to describe something that occurs gradually or slowly. However, there are several synonyms for this phrase that can be used interchangeably to convey a similar meaning. One synonym for creep in is the phrase "seep in" which refers to something slowly entering or spreading into a particular area. Another synonym is "filter in," which suggests a slow and gradual movement of something through a small opening or a system. "Infiltrate" is another suitable synonym, which connotes a covert or stealthy entry into a place or system. These synonyms can be used in various contexts to evoke different emotions and meanings, while still conveying the idea of something gradually entering or spreading.