I know if using Subbly's website builder GTM can be added easily. However, since it requires scripts to be added in both <head> and <body> tags, it cannot be accomplished on our own in the back end of Subbly if we are only using the Subbly's Checkout module only.
GTM will also allow others to add other tracking like Hotjar, Facebook pixels etc via their GTM account, thus keeping the Subbly interface clean. We can switch off/on tracking and test ourselves in GTM. So no need for excessive feature request when tracking can be done via GTM