I recently purchased a TCL 75S551TG Google tv, and wanted to install the Roku app on it. No amount of searching shows the app being available for the tv. Both the TCL and Roku say show the app being available through Google Play but no such luck. Plugging in my ancient Roku stick to the hdmi port, but wanted to avoid going that route. Might there be a geographic restriction here? Based in Canada.
I assume you are referring to The Roku Channel app, not the Roku control app. I didn't think it was available on Google TVs (I thought Samsung TVs were the only one), but I do see it in the Google Play store (play.google.com). However, are you certain it's showing as available on your specific TV? I don't have any Google TVs, but do have several Android TV based devices and they show as supported. The Play store should show if it's actually available for your TV, and if it is you can install it from the web page.
There is a Roku Channel app for Google TV. I have it installed on a Walmart Onn Google TV 4K Pro. Sometimes, the search is a pain as it often gives YouTube finds instead of apps. Try searching for "The Roku Channel app", or just "The Roku Channel".