There is the lerp function.
http://docs.godotengine.org/en/3.0/classes/class_@gdscript.html#class-gdscript-lerp
However, this doesn't provide a timer to as to how long you need for for the weight to go from 0 to 1. If you're using physics_process for an object, you can have the weight be myWeight += 1/(60*myTimeInSeconds)
An alternative is the tween node.
http://docs.godotengine.org/en/3.0/classes/class_tween.html
Please note though that how it works in Godot 3.0 is kind of a mess at the moment (but it will be cleaned up in Godot 3.1).
