Why? Just because they have played alongside wings that are smaller than them? They are the two tallest wings on the whole roster, so it is impossible for them to play alongside taller wings that would help them fit into the retaded category of what cons utes a "shooting guard".
When I see Anderson or Gay as the pick and roll ball handlers at the top of the key, just like Manu has done for years, in what position should I place them? As SF, SG, PG?
And the difference between Paul/Forbes and Anderson/Gay is purely in terms of skillsets and physical attributes. It isn't a difference of "positions".
Anderson and Gay are kind of playmakers, so in offense they will get the ball on their hands. Forbes and Paul are more of spot up shooters (although Forbes has shown flashes of being a good enough scorer).
Then on defense, Gay and Anderson's height allows them to have more of an impact on that end of the floor and guard a wider variety of opponents.
Playing three small wings together would probably not be very effective, that's why it's smarter to split the minutes of Gay and Anderson. But that doesn't mean that a lineup containing both Anderdon and Gay wouldn't be effective, in fact probably the opposite.
Seriously now, do you think a lineup of Tony/Mills, Kawhi, Anderson, Gay and Aldridge would struggle playing together? Of course not.