In the Sharegate Manage portal, you can click the Teams with guests or shared links to review button, select the given group(s), and then click the Ask owner(s) button. This sends an email to owners that looks something like the attached example. Is it possible to edit any part of the default text "Please review the guest within this team as soon as possible" or the signature "Your IT Admin"? In particular, they want to change "as soon as possible" to "by April 1" or "within 14 days" and the "Your IT Admin" signature to "Your ESO Team", for example.
We'll most likely end up sending the emails from the Policies section > External sharing review, but the email verbiage is the same from either location.
I've attached screenshots of the two ways of sending the email notification for reference.