I don't have GD installed sorry. But ...

• ... positive rotations in math are anti-clockwise. Probably a glitch in the docs?
• ... $ARGV[1] ? $ARGV[1] : 90; are you sure you want this? 0 is false, I'd rather //

Update

From the underlying docs at: https://libgd.github.io/manuals/2.3.1/files/gd-c.html#gdImageCopyRotated

The area is counter-clockwise rotated using nearest-neighbor interpolation.

I'll try to contact the author for clarification. (Update: email sent)