Neither, two questions really.

1. Why would he claim almost a full day of labour to change one glow plug and the GPC?

2. Why does he think that given a faulty glow plug, the GPC also needs to be changed?

Finally, it would be interesting to see if anyone has tried to change the GPs. I found a UK based site where posters seem to think it is doable but tricky.

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2003 4.4i black ext./black int./black headliner (kid's runabout)
2014 535d X-drive, M package, silver/black interior

Sold but not forgotten:
2009 E70 35d, black ext./black int./black headliner (sold 2021)
2006 4.8iS Le Mans blue/cream int./black headliner, SOLD in 2012 sadly...

Other hardware:
2015 Cayenne S (wife's new DD and definitely wroooommmmm)
2016 Tundra 1794 edition crew cab, luxo hauler
2005 Tundra crew cab, weekend hauler. Sold after 150k very happy miles.