I am able to install Debian 9.9. My query is: how does the Debian installer decide whether to use MBR or GPT? I don't see that option while installing Debian.
The type of partitioning to use is determined by
partman-partitioning. The rules depend on the architecture; for 64-bit x86 systems, if the installer detects a Mac or an EFI (which means the installer was booted in EFI mode, not BIOS-compatibility mode), or if the disk to be partitioned is 2 TiB or more in size, it chooses GPT, otherwise it chooses MBR.