So, it's actually pretty easy. You already have pfsense set up, and a domain running on Google domains. Open up the Google domains page and the pfsense page so all informations are readily available.
In the domains list in Google domains, click the DNS icon. Scroll down to synthetic records, and from the drop down menu choose "Dynamic DNS."
Assuming you're running a single site, you'll want to add entries for the bare URL (mysite.com) and the www subdomain (www.mysite.com). The bare URL entry should get an "@" sign in the "subdomain" field, and the other should get a "www".
Click the little sideways caret next to the domain name on each entry, and then click the "view credentials" link. This should reveal the credentials for this subdomain - the username and password.
In pfsense, go to the Services tab and choose Dynamic DNS. Click the little plus icon to get a new DynDNS entry.
Under "Service type", choose "Custom". Setup the interfaces according to your network configuration. In the username field, paste the "username" entry from the Google DynDNS page. Copy and paste the username and password from the Google DynDNS page into pfsense.
In the Update URL field, paste this: "https://domains.google.com/nic/update?hostname=". After that, type the name of the domain you are configuring DynDNS for - leave off the "@" for the bare URL version, but be sure to add the "www" or whatever other subdomain.
In the "Result match" field, copy and paste this: "good %IP%|nochg %IP%". This says that the IP update succeeded if Google's servers responded indicating that there was no change to your client's IP, or if they indicated the change was successful.
For a simple webserver setup, add two entries: one for the base domain ("@" in Google DynDNS) and another for the www prefix. They both use they exact same setup in pfsense aiside from the small change to the hostname field, but each will have its own set of credentials assigned by Google.
A previous version of this post erroneously instructed users to leave off the subdomain in the hostname field in pfsense. Updated 1/31/2015 to indicate that the subdomain should be left off for the bare URL, but preserved for "www" and any other subdomain.