You can only mount one partition at one mount point, but any empty directory on one partition can be a mount point for another partition.
GPT is a partition table and is not used for Linux specifically, but on any computer with UEFI - it defines how to find partitions on a disk, but not how they are formatted.
ext4 is a filesystem - formatting a partition with ext4 means creating data structures that tell the OS where to find files and directories in the partition.