The Gases screen is where you select and edit gases for the dive. The left hand side shows the available mixes, while the right hand side shows what you're currently carrying. Click on a gas from either side then click the 'Add/Remove' button to add or remove gases from your current selection.
Any selected gas can have its proportions changed using the 'O2','N2' and 'He' spin controls. If a completely new mixture is created, you have to option to add it permamently to your 'available' list by clicking the 'New' button.
There are a number of preset gas lists available. Simply select a list from the dropdown and click the 'Load' button. You can save your current configuration into one of the "User" slots simply by clicking the 'Save' button, ready to be loaded at a later date.