You can do that, if you define the screen name (not a string containing the screen, that difference is important) as one of the items of the collection that you use for the gallery in the home screen.
If your collection is local, then you need to add a new item to it. For example, this is the (verbatim) value of the Items property of the gallery in the home screen for an app that I've built to use internally to manage DRI duties (some internal process we have in our team):
Table(
{Title: "Before DRI", Screen: BeforeDRIScreen, Subtitle: "Things to do before you are a primary DRI"},
{Title: "During DRI", Screen: DuringDRIScreen, Subtitle: "What to do during the week when you are the DRI"},
{Title: "After DRI", Screen: AfterDRIScreen, Subtitle: "What to do to ensure a good transition to the next DRI"})In my app I have 4 main screens (there are also others, but it doesn't matter for this example): HomeScreen, BeforeDRIScreen, DuringDRIScreen, AfterDRIScreen. The OnSelect property of the controls within the gallery template is set to the following:
Navigate(ThisItem.Screen, ScreenTransition.Cover)
If your table of contents for the main screen comes from a data source, then you'll need to use some logic on the navigation to choose which screen to go (as I mentioned, you cannot navigate to a screen based on a value containing the screen name - you can consider creating a new item in the PowerApps Ideas board for this feature if you feel that it's important). For example, if the name of my screens came from a connected data source and only had the Title/Subtitle properties, I could have an OnSelect value like the one below:
If(
Title = "Before DRI", Navigate(BeforeDRIScreen, ScreenTransition.Fade),
Title = "During DRI", Navigate(DuringDRIScreen, ScreenTransition.Fade),
Title = "After DRI", Navigate(AfterDRIScreen, ScreenTransition.Fade))