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The practical case is more complicated...
GPT uses the beginning and the end of the disk to store metadata.
In general, there's enough free space at the beginning of the disk (i.e. before the first partition) for gdisk to store the GPT (otherwise, see "Head-shrink trick").
However, usually, the OpenXT LVM and partition are configured to span to the very end of the disk, leaving no room for the secondary GPT table. We need tricks to make roomtable (see "Tail-shrink trick").
Head-shrink trick
Luckily, even when the OpenXT partition starts at the very beginning of the disk, the LVM PV starts 2MB after.
The should move the partition 1MB forward, keeping the LVM PV at the same place on the disk, 1MB after the beginning of the partition:
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