is it possible to add plus 1 to a value in gui button?

I do recommend you read everything in that link, but to make it easier for you, the only thing you need to know in regards to your question (using pseudo code) is this:

In a local script:

local RE = game.ReplicatedStorage.RemoteEvent

RE.OnClientEvent:Connect(function(specified_gui,value_to_add)
    specified_gui += value_to_add
end)

GUI_Button_Pressed:Connect(function()
    RE:FireServer()
end)

In a server script:

local RE = game.ReplicatedStorage.RemoteEvent

RE.OnServerEvent:Connect(function(Player)
    RE:FireAllClients(gui,1)
end)

Basically the idea is that when your player presses the map vote button, it fires through the remote to the server, and then the server fires all clients to update their guis of the vote in progress. Just make sure you have the remote event, and rewrite this using your variables and events, and it should work.

I would love to give you more than this, but you didn't make an attempt of your own, so this is as much as I can give you. And if this helps answer your question, then mark it as the accepted answer, and if not, then I'd be happy to answer any further questions you may have about this in the comments below.