My name is Silvija, I'm a part of Omnisend team and I hope this information will be useful for you.
Several aspects contribute to the delivery of your emails. First of all, it is recommended to change your sender's email address to the one on your store domain, instead of Gmail. Please, check this article for more information. We would also encourage you to contact our Support team via in-app chat or at firstname.lastname@example.org so that we could review other aspects, including your customers' engagement, the content of the emails, and technical set-up.