Bug#197469: comment on bug report
the author of gcal decided (according to general practice) to look for the
following environment variables:
If one of those is defined, the value is further examined. There are only
two options: the value starts with "en_" or something different.
The first case means some kind of American style calendar where the days of
the week are written in a top row and the week starts with Sunday.
The second case means some kind of ISO calendar with days on the left side
and the week starts with Monday.
According to ISO/IEC TR 14652 usage of $LC_TIME is problematic and might result
in unintended behaviour. Further it is correct to use $LC_MESSAGE to decide
upon the language of the output. Please keep in mind that gcal does not only
output a calendar but also holidays and events.
So from my point of view the behaviour of gcal corresponds with the
appropriate standards and your bug is rather a feature.
Even the criticized rotation is done on purpose.
In any case, if you want to avoide -s1, you could still set $LANGUAGE.