Why is apex arrow a gcd??

[Rika007]

Because a 600 potency OGCD would be too much. Same thing going on with Saber Dance on dancer.

SMN: Aka Morn and Revelation 650 potency, both oGCD's
DRG: Stardiver 600 Potency, oGCD
SAM: Hissatsu: Gurren and Sensei, 850 and 1,100 potency respectively, both oGCD's (but share a recast timer)
GNB: Blasting Zone 800 potency oGCD.

Not to leave out the already mentioned Automaton Queen and Living Shadow.

The potency of the skills have nothing to do with it.

[Karshan]

SMN: Aka Morn and Revelation 650 potency, both oGCD's
DRG: Stardiver 600 Potency, oGCD
SAM: Hissatsu: Gurren and Sensei, 850 and 1,100 potency respectively, both oGCD's (but share a recast timer)
GNB: Blasting Zone 800 potency oGCD.

Not to leave out the already mentioned Automaton Queen and Living Shadow.

The potency of the skills have nothing to do with it.

For SMN it's pet potency, so it's actually 0.8*650 = 520, but yeah I agree with everything you point out, 600 is definitely not too strong as an oGCD