You have declared the signal speed up in the global manager node (script). This means that game manager is the “owner” of the signal and as such it will only appear in the inspector when you select the game manager node. To sum up signals appear in the node where they are declared.
If you want to connect the signal graphically you have to go to the game manager node, click the speed up signal and then a dialog will appear to select the node you want to connect the signal to (the pipe).
However, I think what you want might be a different thing, correct me if I’m wrong. You want a single game manager node that fires up the speed_up signal and then every pipe receives that signal.
With the setup that you have right now, you are creating 1 game manager for each pipe, so the first thing you need is to create a GLOBAL game manager scene. You can check how to do it here.
Once you have created the global scene and given it a name you can reference that name in your script and connect it as others have suggested. Imagine I used the name GlobalGameManager
func _ready():
GlobalGameManager.speed_up.connect(method)
func method():
# your code here
pass