Is it guaranteed to get entry tickets to the jiuzhaigou natural park in October?

Don't worry! It is guaranteed that you can get the tickets. The scenic spot starts to sell tickets at 08:00am in travel peak season and 08:30 in slack season. If you really worry about that ticket availability, you can ask your hotel to help you with the ticket purchasing.

BTW, October is one of the busiest months for traveling there, you should avoid visiting the place during the golden week holiday (1st Oct. - 7th Oct.). It will be super crowded.