I looked into this and could not find anything definative, especially this blog. The best answere were that they were just marketing terms (ESX and GSX) and the other possible one was related to the possible intended audience. Enterprise Server X and Group Server X, no one however said what the X stands for.
I imagine it didn’t stand for much of any thing, like NT and XP, because they renamed it at version 3.0 to VMWare Infrastructure.
ESX is still the name for the barebone system, Virtual Infrastructure refers to a suite of products used together, ie: virtual Center and ESX forming a virtual infrastructure, got my backside kicked at a VMware seminar when I called ESX Virttual infrastructure. save someone else getting the same treatment!