PCB123 is not intended to be a gerber viewer. It will allow you to import gerbers but it makes changes based on the available PCB123 options and there are two issues to account for at this stage.
1. PCB123 gerber import will not work at all for a multi-layer board with internal plane layers. Negative polarity plane layers will not import correctly.
2. The PCB123 gerber import tool will often change a surface mount pad to a through hole. This places a pad on both sides of the board and a hole in the pads connecting the two sides of the board. You will need to manually fix this issue before submitting your order.
I do not recommend using PCB123 as a gerber viewer. PCB123 will change features based on the built-in design rules and a limited number of available hole sizes. If you need to make changes to gerbers and do not have a gerber editor you can import and modify gerbers using PCB123. Once you import gerbers into PCB123 you can save your file in PCB123 format. If you just need to review your gerbers, there are many free gerber viewer programs on the web. One you may want to try is http://gerbv.sourceforge.net/ .
We plan to correct the reported issues with gerber import in a future release of the PCB123 software. However, as I mentioned PCB123 will change your artwork so unless you need to modify your gerbers, you should use a third party application to preview your gerber files and order directly through the sunstone.com website with your gerbers.