When a graph is active, selectGadgets:Quick Peaks...orGadgets:Integrate....
You can fit your data, choosing theAnalysis: Fitting: Single Peak FitorAnalysis: Peaks and Baseline: Multiple Peak Fitmenu. When fitting with the Gauss function, the FWHM is output to the report. If your function is Gaussian (note that it is different from Gauss) or Lorentz, the w parameter is the FWHM.
You can also selectAnalysis: Peaks and Baseline: Peak Analyzer.