In Geany, is it possible to indent multiple lines at the same time using a keyboard shortcut (instead of typing every indent by hand)?
Maybe I did not understand the question correctly, but both the TAB key and the CTRL+i shortcut do exactly that when multiple lines are selected.
For unindenting multiple lines, you can use both the SHIFT+TAB and CTRL+u shortcuts.
Generally, (SHIFT+)TAB seems to work in 95% of all modern text editors. CTRL+i/u is more Geany-specific, but can easily be found in the Geany Settings (Settings -> Keybindings -> Format).