Yes, you can save a Gmail mailbox as a PST file, but Gmail doesn't create PST files directly. One option is to add your Gmail account to classic Microsoft Outlook using IMAP, let the mailbox sync completely, and then use Outlook's Export to PST feature. Just make sure Outlook has downloaded all your emails before exporting, otherwise the PST may be incomplete.
If you're looking for a simpler approach, I've used the BitRecover Gmail Backup Wizard. It exports Gmail directly to PST with modern OAuth login, selective folder and date filters, incremental backup, bulk account support, and preservation of attachments and folder hierarchy.
One useful feature that isn't mentioned very often is that it can split large PST files into smaller sizes during export, making them much easier to open, archive, or import into Outlook later.