Probably switch to another hosting provider.
If I read this from Hostgater correctly: How Do I Use the Third-Party SSL Certificate I Purchased?, it basicly comes down to this:
- Elaborate procedures due to difficulties at Hostgator to get a certificate from Let's Encrypt (cóuld be very easy, but they decided to make it difficult);
- Pay Hostgator a fee at a minimum of $ 10,- (for a single domain) to install the certificate.
See Web Hosting who support Lets Encrypt for a list of (un)supported hosting providers. You can find Hostgator in the "Waiting/Delayed" section.