Hi @luke500,
A warm welcome to the Bunnings Workshop community and thank you for your question.
I'm not 100% certain whether it's grout, or it is silicone.
If you poke the existing filler beside the gap, is it soft and flexible? Or is it hard?
If you find it is soft, it is likely they have used silicone, in which case, you should use Sika White Premium Swimming Pool Silicone Sealant to fill the gap. Check out How To Silicone a Gap for some guidance on this process.
If you find it hard, it is likely grout such as this Davco Rejuvenation Grout. You can use a Kango 10mm Tuck Pointer to push the grout into the gap, ensuring it is compressed and has a good connection with the top and bottom of the opening.
Let me know what you think and if you have any further questions, please don't hesitate to ask.
Jacob
