You cannot install a source archive directly with pacman - the -U argument requires a valid package created with makepkg.
Buildscript is available from AUR - AUR (en) - gcal
pamac build gcal
Then
$ gcal -s 1 -i -K -q AT 1, 2022
January 2022
Monday 3 10 17 24 31
Tuesday 4 11 18 25
Wednesday 5 12 19 26
Thursday 6 13 20 27
Friday 7 14 21 28
Saturday 1 8 15 22 29
Sunday 2 9 16 23 30
Week 52/0 01 02 03 04 05