Umm
That's not even counting the extra stats she gets out of her passive and the active which is a big fucking deal, not a single stat is wasted and she makes so much out of the reset passive on the item's active. Gunblade gives her a massive power spike in damage thanks to the extra burst she gets from all the stats and active, dueling power thanks to all the sustain she gets and lets her postpone or ignore completely Rylai's thanks to the active. There's hardly anything inefficient about it, specially when the item has an amazing build path for Akali with components that range from 360 to 1400 gold and help her laning a fuckton, and once she gets nerfed next patch she'll need the lifesteal/spell vamp more than ever.
Also I find it hilarious that you use diamond players as an argument but completely diss challenger players building her in solo Q because of some bizarre competitive prejudice.