Oh Jimmy boy! am surprised ye didnae ken that wan, Siding? is a daft US term for exterior wall cladding, this is the exact description from the US Jester dictionary.
GI roof sheets are just Sh!te, the gutter system that goes with it and the methods employed are terrible, as far as I know the way they transition the squared gutter to the round downpipe is by forming a bit of the scrap sheet metal into a round with a Pop Riveted seem, this is so very bad and I can only guess there is not a preformed part for this application?
The answer to all of the above problems is to use the IMAC roofing system, available from DN Steel, this is uPVC plastic roofing of very high quality, little of any heat passes through it, I know this as I used this on the roof of the wee Hoose I built there, it is a more resilient material than GI steel, me being a Fat Fecker I can jump on the sheet and find it impossible to distort the profiles or even dent it, with the right design this provides a very cool roof and requires no insulation, it does not rust and a lot less noisy in the rain, they also have a fit for purpose uPVC gutter system that goes with it, this is a well designed product originally intended for use on commercial buildings.