Gunblade is a lot of AD, and she gains bonus magic damage on AAs, so attacking fast means she hits like a truck (and can pop Q nearly instantly). CDR makes her ult come up at times where it wouldn't for non-CDR Akalis.
I agree, though, that it's hard to find an item to replace it. Gunblade/Rylai/Deathcap/Lich/GAkali is very strong (Substitute Zhonya and DFG as desired), but very energy-dependent. Going Nashor's would make her more AA-reliant, and while they do hit for a ton, and can hit very quickly, it may not be worth the sacrifice of her up-front burst.
Against a tanky team that can't be bursted as hard, though? It may be more viable. I'll have to math it out. --- If inflaton turns $3 into $0, then $6 turns into $3. -Orca