Since stencyl 3.4 there are problems with gradient variants
C:/Program Files (x86)/Stencyl/plaf/haxe/lib/stencyl/1,00/com/stencyl/models/background/GradientBackground.hx:35: characters 51-57 : Array<Int> should be Array<Float>
GradientBackground.hx:35: characters 51-57 : Type parameters are invariant
GradientBackground.hx:35: characters 51-57 : Int should be Float
/background/GradientBackground.hx:35: characters 51-57 : For function argument 'alphas'
so I corrected it by replacing var alphas = [100, 100]; by
#if js
var alphas:Array<Float> = [100, 100];
#else
var alphas = [100, 100];
#end