To boot with BIOS you do not have to change to MBR. Windows requires you to convert to MBR if booting with BIOS but Ubuntu/grub boot just fine from gpt. This is my flash drive with gpt partitioning and I boot with BIOS. The only added thing is you need to create a bios_grub partition of 1MB which can be anywhere on drive. Turn off ef00 or boot flag on sda1, your efi partition so grub does not think it still is UEFI.
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Code:
red@fred-LT-A105:~$ sudo parted /dev/sdb unit s print
Model: Kingston DataTraveler 2.0 (scsi)
Disk /dev/sdb: 31375360s
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Number Start End Size File system Name Flags
1 2048s 4095s 2048s KingstonData bios_grub
2 4096s 14749695s 14745600s ext4
3 14749696s 29327359s 14577664s ext4
4 29327360s 31373311s 2045952s ntfs
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