When you install the Recompose Gmail add-on, the final step asks you to allow a list of Gmail permissions. While granting these permissions would technically allow use of your private information, rest assured that Recompose will do nothing more than create the drafts you request.
None of your information is ever stored or shared, for any reason.
Recompose's use of information received from Google APIs will adhere to Google API Services User Data Policy, including the Limited Use requirements.
The Recompose add-on is provided "as is", without warranty of any kind, express or implied. In no event shall the author be liable for any claim, damages or other liability.