Gerber data is "unintelligent" (simply X,Y coordinates) and
does not contain any component information (like ODB++ and
IPC-2581). The gerber file shown on your screen capture has
components merely drawn as individual trace segments (no component
information). If you perform "Component check" on plain Gerber files, FAB
3000 cannot find any component errors since there are no defined components (only traces).
If you're interested, you can generate components from Gerber files using FAB 3000.
However if you happen to have an IPC-D-356 netlist file (which is typically included with Gerber files for Net Compare and bare-board testing), FAB 3000 can easily generate centroids from that netlist file and determine spacing. Here's a quick tutorial showing me perform the following: