From the function you wish to make the call get the node and use one of that nodes call_method function (call, callv, call_deferred
)
Click Add Nodes..
Change
- Call Mode to Node Path
- Function to get_node
- Node Path to the node with the GDscript

Click Add Nodes..
Change
- Call Mode to Instance
- Base Type to Object / Node
- Base Script to the path of the GDscript with the function
- Function to the function you wish to call

Here i use the override _ready():
func
