As Shakespear has posted above, that method will work. In case you were struggling to make sense of it, the WELLWAG keyword is the 'special sauce' that would make your life easier - look it up in the Eclipse Reference Manual.

In the unlikely event that you wish to do a E100 run with this sort of alternating process, another workaround is simply specify two identical wells for each injector - one as a water injector, other as a gas injector and then alternate production/shutin between them with 30.4 day timesteps.