Never heard of this brand, they look like they would only draw a very slight vacuum.
Some of their equipment looks like it might only be a vacuum blower and not an actual vacuum pump.
Also 15" hg is not a very deep vacuum at all, are you sure this final pressure is sufficient for your application?
Two big vacuum manufacturers I have come across are Busch & Edwards.
I recommend researching their oil flooded rotary vane pump range.
They will probably be more expensive then GSE however.