Nothing wrong with using a Gmail address. Many sole traders use free email services. Do you have the engineer’s Gas Safe register number? You should see his/her/their credentials before work started on your boiler. You should be able to search for at least the company name on their website. Once you have the engineer’s name and Gas Safe number or the company’s Gas Safe number, go back to the engineer and let them know that they have two days to come back to complete the job before you report them to Gas Safe.