So, here is how I resized my ext4 root partition (sda2) using gdisk and resize2fs:

$ gdisk /dev/sda

Print information (i2)
Partition GUID code: 0FC63DAF-8483-4772-8E79-3D69D8477DE4 (Linux filesystem)
Partition unique GUID: 7FAC4BC8-6186-4CDF-B6CA-EB73D7C942D1
First sector: 206848 (at 101.0 MiB)
Last sector: 109051903 (at 52.0 Gi...

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