Hi,
You can find 3rd party, pre-built Kraken2 16S GTDB databases in the third-party tools section of the GTDB website:
https://gtdb.ecogenomic.org/tools
If these existing databases don’t fit your requirements, there is also a script linked to on this page for converting the GTDB taxonomy to NCBI-style taxonomy files.
Cheers,
Donovan