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Is it just the gear knob itself that's broken, or the shaft as well?
Knobs can be easily changed. 2nd hand ones are available via ebay and the like.
This is for the Insignia, but the process is the same for the Astra.
https://www.youtube.com/watch?v=bNDy8QS5E44&t=12s
The only problem you might encounter is, if as you say the button to engage reverse and park has come off, I'm not sure how you're going to press it in to remove the knob.
Knobs can be easily changed. 2nd hand ones are available via ebay and the like.
This is for the Insignia, but the process is the same for the Astra.
https://www.youtube.com/watch?v=bNDy8QS5E44&t=12s
The only problem you might encounter is, if as you say the button to engage reverse and park has come off, I'm not sure how you're going to press it in to remove the knob.