Yes. I, in fact, have three pension files for veterans who lied about their age and their names (i.e. used aliases). The only requirements were to have served honorably for at least 90 days between 1861 and 1865 (forget the exact dates just now). The only exceptions I know of were a few questionable Missouri militia units that weren't allowed pensions (there was a committee that reviewed Missouri militia units after the war to determine which were actually valid for pension purposes). Members of Oregon units that didn't fight in the war at all - i.e. fought Indians at most - were eligible and could join the GAR.