Most guides I've seen to builds in SCII I see a list of items next to non-sequential numbers. Example. What do the numbers represent?
It tells what your supply count is at when the command to create the item is at.
this means, if you have used 13 supply (which means 13 probes), build your gate(way) then.
Elsewhere in the linked answer there are messages like:
2 Gate Rush
Which means, rush with units after you have two gate(way)s
The reason that the numbers in the answer that you linked to are not sequential, is because the list is not a build order. It is a list of the names of several opening strategies (or "openings"). McKay's explanation about supply count is correct when talking about build orders, but these are not build orders. "2 Gate" in this case simply means to open with 2 Warp Gates--it doesn't specific the exact build order. "3 Gate into Robo" means to open with 3 Warp Gates, then build a Robotics Facility.