If the email address you used to make the purchase is not @gmail, and you don't have a google account associated to it, there's 3 options to gain access:

Option 1

Reply to the purchase confirmation email that you received from Gumroad with your gmail address and I will add access to it.

Option 2

Alternatively, you can also add your Gumroad email as a secondary email for your google account:

Sign in to your Google Account with another email address

Option 3

Create a new Google account, using your Gumroad email address. You can do it by going here, and selecting the option highlighted in the following screenshot: