When sending to Gmail, you can do your best to ensure deliverability by making sure that you get a 10 out of 10 score from mail-tester.com or a similar email deliveraiblity testing service. To do that, go to mail-tester.com, copy the email address given to you, send an email and then click the button to check your score.
It may also be possible that the recipient has marked your sending address as spam at some point.
More information can be provided by Google: Prevent mail to Gmail users from being blocked or sent to spam – Gmail Help (google.com)