STBRUB is right and your captains are wrong (in fact I am amazed at the lack of basic understanding from what you claim are senior captains)
Neither the ADIRU (whether you have a single intergrated unit or 3 individual units) nor the GPS can navigate. they can only tell you where you are (and in the case of ADIRU's only if you accurately tell it where you started)
An FMC however, uses data provide by the ADIRU or GPS to navigate the a/c along a determined route.
Many of the more modern a/c have 2 FMC's (some even a 3rd) which satisfy the long range requirement. Often, those with only 2 FMC's have an MFCDU which has an integral back up nav computer built in so in the event of an FMC failure there are still 2 long range nav computers.