I see that we can get Pureflix but not GMA. Is there a reason?
You'd have to ask Great American Family. Roku is just a platform; for the most part they don't create the apps.
You could contact GAF customer support and ask them to create a Roku Channel. If they have enough demand for it, they may create one.
The cheapest way to get Great American Family is to subscribe to FrndlyTV on your Roku device.
https://channelstore.roku.com/details/300d1f91b5e583145c8a3fba6f08dafd/frndly-tv
https://try.frndlytv.com/ (Frndly website - channel list)
-----
From the GAF faqs page, https://www.greatamericanfamily.com/faq/, you can also see the other cable-alternative packages that offer it. (Fubo TV, Philo, Hulu, DirecTV Stream and Sling TV).