What is most surprising about the notice on the website is the ambiguity - it's entirely up to the average reader to decide whether it affects Gawler station or the entire length of the line! Looks like somebody forgot the "check it makes sense" step before publishing it.
The reason stated on the website notice is "track maintenance" which could well mean anything.
If it is to be departing from Elizabeth on the wrong track, that would have it running on the wrong line as far south as the approach to Salisbury thanks to there being no crossovers* between the one north of Salisbury and the pair just south of Gawler. If that's the case then the second-last train to Gawler will have to cop an eight minute delay at Salisbury while it waits for the wrong line train to get to that point.
There's nothing wrong with trains departing Gawler from platform 1 as all three platforms have the signals and necessary crossovers to go both ways. If it was just Gawler though, a notice probably wouldn't be necessary at all since platforms 1-2 are the opposite faces of the same physical island platform. What happens with level crossings between there and Salisbury could be interesting though.
* before anybody points out the turnback siding north of Elizabeth, that has never been in service. It might count as "in service" only by the measure of David Peters though, along with the likes of the Roseworthy, Mount Gambier and Appamurra lines.