Philips head screwdriver, #PH000
You would also do well to get a spring bar tool which isn't expensive but makes removing straps a lot easier. Nothing difficult with GA110 or 100. Make sure when you remove screws to note which shape comes from where as you may find different kinds in bezel, back and wrist protectors. Always good to take photos if you are unfamiliar with a watch.
Screws love to break for freedom when they are loosened. Best to contain their enthusiasm by working with hands and watch inside a large plastic container - like a (dry!) washing up bowl. Then, when they do try to escape they find it very difficult to climb the walls and lie sulking and easily spotted in the bottom of the container. Don't be lulled into using a ceramic or metal bowl BTW, screws use that kind of material as a trampoline.