[syslinux] Syslinux boot from a partition on sub stick with GPT label

[Fulvio Scapin]

I recently tried to install several distros on a usb stick, with the
peculiarity of creating in it a GPT label rather than the usual dos label.
Installed Grub2 using the parted bios_grub flag on a partition, I proceeded
to copy several distros on the remaining partitions.
Grub2 worked fine, chainloading syslinux on each of the partitions. Or
rather it worked all fine until I reached the fifth partition. Once tried
with that all I got was a boot error.
Having tried extlinux on that one I persisted thinking that might have been
the source of the problem. So I duplicated the contents of the fourth
partition, installing syslinux as with that one.
Boot error, again and again.
The fact that the first four paritions work as expected sounds suspiciously
related to a situation involving the use of a dos label to me. Could anyone
explain to me where the problem lies:
with syslinux and gpt, with grub2 and gpt or somewhere else?
Thanks in advance and forgive me should I have wandered off-topic.

Fulvio Scapin