Most of the Gawler line is deep red (Enfield, Port Adelaide, Playford, Ramsay, Elizabeth, and Taylor.
So deeply red that even ALP governments don't need to chase votes there.
If they had a better history of holding their MPs accountable, the Rann/Weatherill government might have tried a bit harder with their false starts on Gawler line electrification.
The only areas where there may be some political fallout would be the start of the line (Adelaide) and end of the line (Light). Light has a strong Labor local MP in Tony Piccolo - if anything the delays will make him safer. That leaves Adelaide - which is the only Liberal held seat on the line. Given the line from Salisbury to Adelaide will be out for a good 15+ months by November, this could make Adelaide a little vulnerable for the Libs (if Labor play the, 'Libs are so bad they shut your train line down for over 15 months' card).
It is wildly optimistic to assume it will have any impact on Adelaide.
I'd bet that any impact of the GREP delays would only partially offset the own goal scored by Malinauskas when he announced he will bring the taxi races back to the parklands.
When the Noarlunga/Seaford line was subject to similar degrees of mismanagement in 2013-14, it made no impact whatsoever on the seven ALP held seats along the line. Five were retained, and the two that were lost (Bright and Mitchell) were already destined to be lost before the rail project delays ever started.