Please help player drop down menu in gui?

Hello I'm trying to figure out how to get all players connected on a server too show up in a custom made gui?

Like for example. A drop down menu. You click that shows a list of all the players currently online and you can select one. Then everything else you select in the gui is for that player you selected only.

So lets say I make me a custom admin gui for my server.

In that gui I got all kinds of commands.

I click my dropdown menu in my custom made gui select "DonaldBuck" whos currently online and connected to the server then click a button called 'God mode' and it makes only him have god mode in the server

thanks