Gerber doesn't natively support text objects. The reason I mention this is Text editing may be a bit messy with gerber files because you're not actually editing text objects but just a series of traces, polygons, etc.
Text in most Gerber files is going to be a series of traces (i.e line draws) or polygons, and not an actual text object (with properties like origin, text string, font, alignment, etc). You can determine by moving the mouse over the text object -- if it displays "Trace" than your text is just line draws.
Note: I believe the reason for the grayed out command is that the object you're editing/selecting is not an actual Text object, but just a series of traces (i.e line draws).
Here's a quick movie (no sound) showing me work with both a text object and gerber text drawn as a series of traces in FAB 3000,
Note: I turned the fill off at the beginning so you can see the series of traces (i.e lines) used to draw each letter. I measured the text height first and then chose a suitable font (from the list) to use. Can't find the perfect font? You can import any TrueType or Shx font to be used by FAB 3000, https://support.numericalinnovations.com/support/discussions/topics/14000003029