Gab also used Mastodon but they don't anymore anymore. They did run as a Mastodon instance[1], but after being blocked from most instances and even at the software level by most Mastodon apps, they stopped using Mastodon and wrote their own new backend that does not federate.
Gab has also been banned from Fdroid[2].
[1] https://www.theverge.com/2019/7/12/20691957/mastodon-decentr...
[2] https://reclaimthenet.org/f-droid-bans-gab-app/