I wouldn't call its GUI really cross-platform yet. Linux GUI definitely isn't production ready. Android is somewhat usable. iOS is not working.
Nevertheless, I keep my fingers crossed. My estimate is in 2-3 years you will be able to start writing small cross-platform GUI apps in RED and not worry about trifle problems.
One major stumble block in GTK backend development is a monstrocious string-oriented API for setting widget properties. There's a more low-level one, but it's depricated.
FYI, development of backend happens as we speak [1].
This came up not long ago, and we have to define "cross platform" to say whether Red meets the criteria. Red does have a cross platform GUI system, though it is only fully implemented on MacOS and Windows currently. We don't run on iOS at all yet. What the current system shows is that a single, cross platform dialect can abstract away the native OS APIs in a clean way, though there will always be tradeoffs, as every OS has different features.
The RED Wallet is an example of a cross platform app, done today.
Nevertheless, I keep my fingers crossed. My estimate is in 2-3 years you will be able to start writing small cross-platform GUI apps in RED and not worry about trifle problems.