PCB123 does not directly allow you to import gerber files for review so we have provided the Print Artwork tool to help you review your layout before you submit your order. The Print Artwork feature is found under the File Menu File Print Artwork. This will allow you to print individual or a combination of layers for review.
Select the layer(s) you would like to review. You have the option to fit the print out to page, scale the print or mirror the printed layers. To save paper you may want to print to a pdf document. PCB123 does not provide the ability to print to pdf, but you can easily download and install a pdf printer driver. I like cutepdf. It is free and very easy to use. Once you have it installed you select it just as you would a standard printer and print to pdf.