From the past experience, they are not very useful and have compulsory attendance. As of 2013-2015, there is no choice in them. This is a workload estimate:
1) Intro to Thesis Writing: lecture time (24h) + coursework (2000-word survey in your research area)
2) Research Ethics: lecture time (12h) + essay (500-words about ethical issues in your research area) + online course (~1-2 days fulltime clicking and reading through the online course)
3) Transferable Skills: lecture time (12h) + coursework depends on the lecturer (e.g. some students reported a heavy workload where they had to hand-in a short essay after/before every lecture; in my case, it was just one document + short presentation at the end)
4) Intro to Quantitative Research: lecture time (12h) + coursework (up to ~4-6h: elementary level statistics problems and essay about statistical analysis in a paper in your research area)
As of 2013-2015, Research Ethics was offered only once per year, so one had to be careful not to miss enrolling in it because of probation requirements.
CETL (compulsory teaching course) is similarly time-demanding as GS courses.
If you don’t have major teaching duties, 2 GS + 2 departmental courses should be reasonable to choose in one semester.