The reason for this is because the GuiObject.ZIndex property holds an integer value, and you attempted to compare it to a string, which will return false. Simply make it -1 and it will be good from there.

Truthy values

In Lua, anything but false or nil is truthy. So numbers, strings and tables are all examples of truthy values. The boolean true itself is also truthy. If you print(not 5), that would print false, as you're inverting the "truthiness" of the number. So by you doing if not v.ZIndex == "-1" then, you're actually just saying if false == "-1" then. In order to fix this, you can wrap the v.ZIndex == -1 in brackets, or make your == an ~=.

if not (v.ZIndex == -1) then

----OR----
if v.ZIndex ~= -1 then

not only works for booleans, not if the condition is not met.

Try this instead. ~= means not equal to.

I quit the "", because the ZIndez is a number, not a string.

if v.ZIndex ~= -1  then
    v.Visible = false
end

Hope it helps