Hi Dukki,
Looks like the gates are available, but they aren't named correctly. Gate 3 = C3 and Gate 4 = C4. Thanks for spotting, we'll note it down to fix.
As for gates that don't show up at all, any gates that have a static aircraft won't show a start option. If you want them to, you have to disable static aircraft in the LYBE control panel, which you can access through it's product page on Orbx Central by clicking 'Configure'.
Cheers, Misha