Gerber Analyzer is described as 'Opens PCB Gerber files in the browser and goes beyond plain layer viewing. It runs thermal, EMI and impedance simulation directly from the board data, so you can see where copper is likely to run hot, where EMI may be a problem, and whether controlled-impedance' and is an website. There are five alternatives to Gerber Analyzer, not only websites but also apps for Windows and Linux. The best Gerber Analyzer alternative is EasyEDA Gerber Viewer, which is free. Other great sites and apps similar to Gerber Analyzer are ViewMate, HQDFM, 3D Gerber Viewer and Gerbv.