You can book an appointment as soon as you have registered with g.a.s.t. For the booking, you need a transaction number (TAN), which you receive from your test centre.
The deadline may vary. Please refer to the website of the test of your centre.
Yes, provided they are in different languages. However, you may not book more than one appointment in the same language in advance. If you wish to repeat the assessment, you will have to take the test first. After completion of the test you may again book one appointment per language.