Godbus

GPU stands for Grid Performance Unit.
A GPU is a measurement that derives from CPU time required by every single hit/request made to your (gs) Grid-Service. This includes your web server activity, programs, scripts and all associated parent and child processes across all nodes within our system (100′s of processor cores).

The GRID system keeps track of all the measurements taken from your server and adds them to a running total which is calculated about once an hour.
A GPU (Grid Performance Unit) is equal to:

• 1000 GPU’s is equivalent to running a continuous 10% load on one processor for the month.
• You can use up to 1.38 GPU’s per hour and not go over the 1000 GPU limit
• 1 GPU = 7.24% of 1 CPU for 1 hour

  NOTE: The above numbers are based on a 30 day calendar, these numbers will vary based on the actual calendar days for each given month in your billing cycle.

If you should exceed your GPU allocation you will simply be billed at the end of the month for the overage that your server incurred at the rate of $0.10 per GPU.

Get more information about GPU from http://mediatemple.net/webhosting/gs/faq.php?category=63.