I bought my Gaslow system secondhand. The guy who sold it to me had fitted an underslung tank again a Gaslow tank. We discussed the differences between Gasit and Gaslow. The outcome was Gasit was a cheaper option and the cylinders where basically the same as Gaslow.
But when it came to the hoses, fittings and filler connector the Gaslow stuff was of better quality the hoses being braided steel etc.
Personally I don’t think there is much of a difference apart from Gasit being marginally cheaper.