Gunblade is a staple for many hybrid champions, but the powerspike this item offers leaves very little room for actual itemization choice for some champions. Either you can't use it, or you rush it first items. Also the few champions that can actually use it properly gain so much power out of it that they can only be balanced around that item in mind.
As far as hybrid itemization goes, Gunblade does a poor job at actually appealing to hybrid champions. Only bursty AP champions can effectively use it, koodos if they actually scale with AD. Most of them simply build full AP with a gunblade rush as a first item.
I feel like Gunblade should be reworked around actually building both AD and AP items. Maybe give it a passive inspired by the old Akali passive.
Here's an exemple of what it could be:
Unique Passive: You gain 50% of your bonus AD from items as AP. Your autoattacks now deal 10%(+1% per 10AP) of your Bonus AD as additionnal magic damage. You also gain Spellvamp equal to your lifesteal.
Unique Active - Lightning Bolt: Deal damage equal to this item's unique passive (including the base 100% AD) +30% AP and slows for 40%.
Obviously this item would be bad as a first item, but it would scale really well for champions that actually buy both AD and AP items. It would offer no bonus AP without previously buying AD items, and its bonus damage on hit would be very weak without either AD or AP items. A champion like Akali could buy a Stormrazor + Morello then get a Gunblade to increase her item synergy.
If Infinity Edge and Rageblade can be reworked so that it can't work as a first item anymore, i feel like Gunblade should get reworked too.