pfSense

it can be manually configured using the "custom" type currently

I'm aware, but it took a while to figure it out on my own. It would be much simpler to have a menu item of it's own. It's not a priority - just something that would be nice to have in a later release. If I knew programming, and was more familiar with GIT, I'd try doing it myself and submitting it.

https://forum.pfsense.org/index.php?topic=86900.0 explains exactly how to do it as well.

I second that it would be incredibly nice to have a 'Google Domains' dropdown in the list.

The biggest actual issue I can see when using 'custom' is that the hostname does NOT display on the 'Dynamic DNS' summary screen.

Adding a vote for this... have manually added it using Custom, but would like to get full status support (rather than just "good") which isn't possible with Custom DDNS entries. Their service is dyndns2 compliant so it should be a quick add.

An issue I see with those instructions on custom dns entries is that it requires your google domains account credentials rather than the username/password generated for your dynamic dns. I'm in the process of getting this working so maybe I'm wrong..

Since this is the basic dyndns service - just hosted by google's dns servers instead of dyndns - it should be a really easy thing to add.
Here are the ddclient config parameters assuming it's the ddclient that doesn't already have google domains support:
protocol=dyndns2
use=web
server=domains.google.com
ssl=yes
login=generated_username
password=generated_password
your_resource.your_domain.tld

I will happily test this in a beta version.

Incorrect info. The API call does take the custom username/pw and not your account info.

https://redmine.pfsense.org/projects/pfsense/repository/revisions/6c92b37851b4f23c82ce6e7f1094b9db1adc4b44

Looks like someone contributed some code for this feature already. How can I test this?

I see Google Domains as a DDNS option in the latest 2.3 beta snapshot... I'm not in a position to try it at the moment though as I have it running on a VM, not as my main router/firewall (yet).