Thanks @reC! Here is the first partition (the one to start with) corrected to:
gdisk /dev/sd[x]
…
Command (? for help): n
Partition number (1-128, default 1):
First sector (2048-30318558, default = 2048) or {±}size{KMGTP}:
Last sector (2048-30318558, default = 30318558) or {±}size{KMGTP}: +20480
Current type is 8300 (Linux filesystem)
Hex code or GUID (L to show codes, Enter = 8300): ef02
Changed type of partition to ‘BIOS boot partition’
parted /dev/sd[x]
GNU Parted 3.3
Using /dev/sdq
Welcome to GNU Parted! Type ‘help’ to view a list of commands.
(parted) u MiB
(parted) p
…
Partition Table: gpt
Disk Flags:
Number Start End Size File system Name Flags
1 1.00MiB 11.0MiB 10.0MiB BIOS boot partition bios_grub
(parted) u MB
(parted) p
…
Partition Table: gpt
Disk Flags:
Number Start End Size File system Name Flags
1 1.05MB 11.5MB 10.5MB BIOS boot partition bios_grub