Auntie google tells me that 16 gauge is 1.6mm and this is 1.5mm. ERW means 'electrical resistance welded' so it will have a seam and will have much inferior structural properties to a seamless tube (but that might not matter).
88.9mm OD is available in 1.6mm thickness, but again only in ERW. You can also see from the brochure the minimum thickness in which seamless is available (e.g. 88.9 x 3 CHS).