Global variable does not have global scope

The documentation for global variables can be found here:
https://autohotkey.com/docs/Functions.htm#Global

Global variables

To refer to an existing global variable inside a function (or create a new one), declare the variable as global prior to using it. For example:

LogToFile(TextToLog)
{
    global LogFileName
    FileAppend, %TextToLog%`n, %LogFileName%
}

I believe the concept of global, with AHK, is a bit different than in other languages. With AHK you can create a variable and use it within multiple hotkeys, and subroutines, without declaring it as global.

Gv := 0

f1::SetTimer, Action, % (on:=!on) ? (1000) : ("Off")

Action:
    Gv++
    trayTip,, % Gv
Return

f2::Msgbox, % Gv

Explaination of code:

• The F1 key toggles a timer to run the subroutine: Action every 1000ms.
• % starts an expression.
• on:=!on reverses the binary value of variable on every time F1 is pressed.
• ?: together is called the ternary operator.
• When on=1 delay is set to 1000ms; when on=0 the timer is turned Off.

The ++ operator adds 1 to variable Gv.

This makes things easier:

https://www.autohotkey.com/docs/Functions.htm#SuperGlobal

Super-global variables [v1.1.05+]: If a global declaration appears outside of any function, it takes effect for all functions by default (excluding force-local functions). This avoids the need to redeclare the variable in each function. However, if a function parameter or local variable with the same name is declared, it takes precedence over the global variable. Variables created by the class keyword are also super-global.

Just declare your variable as global in the main script:

global supposedlyGlobalVariable := "blah"