Go Shuttle is just a marketing consortium of independent ground transportation companies. It will be a different provider depending on where and what type of service you book. The actual provider name is hidden from you during the booking process.
IMHO, you're better off dealing directly with the individual company, so you know who you're dealing with.
I've seen the GO Shuttle logo on Mears vans around WDW, so I assume you'd actually be booking a ride with Mears in the Orlando area.