For G Suite.
For Office 365.
Is it possible to restrict the access to HDE Secure Transfer by IP address?
No, it is not possible to restrict the access by IP address.
For the users those who have introduced the access control, however, it is possible to put the same kind of access restriction as the one when using mailing function since it is necessary for them to log in to G Suite/Office 365 when using HDE Secure Transfer.