The description suggests there's a Git repo in the challenge web
root.  And indeed, visiting
https://gitgood.2020.chall.actf.co/.git/HEAD doesn't error out.

    $ git clone https://gitgood.2020.chall.actf.co/.git
    $ cd gitgood.2020.chall.actf.co 
    $ git log
    $ git show 6b3c94c0b90a897f246f0f32dec3f5fd3e40abb5 | grep actf 
    +actf{b3_car3ful_wh4t_y0u_s3rve_wi7h}