I've never been a fan of pure damage builds, not pre-rework, not post, because doing so effectively shuts you out of melee range. Closing to melee is vital to followup a keg explosion, or as a contingency plan if your barrels get sniped away (if you can't close to melee in such a situation, it's essentially a 4v5 until Parley comes off cooldown, which you cannot afford).
That said, it seems high-level players (pronounced: not me) have been going straight damage with decent success. Ironically the coming nerfs will hit tankier builds far harder than straight damage builds, as kegs make up a smaller percentage of your damage as you build more attack damage items.
Even though Gangplank lost a lot of his item variety and has been pigeonholed more towards raw damage and critical strikes over on-hit effects, I still feel like he can successfully choose from a variety of items in many different situations. My final build, in no particular order, is usually something like this:
[
/
/
] [
/
]
The first four items are my go-to damage set, and the last 5 items consist of the two defensive items I will build when I deem necessary.
Other, more situational items include, but are not limited to:

I think most of my choices merit some form of explanation on my thought processes.
: I rush this every game. I think every GP does. It's just too damn good not to.
: I think this thing is so underrated on GP. It's not very useful when going full offense, but it shines when the lifesteal can bolster your high HP and protections with a more defensive build. The 80 damage combined with kegs is also quite nice. Not sure why I don't see this more often. I guess most players like a 2nd
more? Personally, I've never liked relying on crits.
: For the love of God, do NOT rush this. Pair it with Trinity. Defensive stats are more well rounded than Randuin's. Pop a 100 charge Q into someone to slow them, then use your speed boost (from the Trinity Force you definitely have) to catch up and slap them with your Fire Sword.
: I've always been an advocate of Frozen Heart on ol' GP. Pre-rework, it let you spam gunshots and oranges faster and for longer. Now it does the same thing, but also grants you faster keg charges. The 90 Armor is lovely.
: 20 flat armor pierce is pretty godlike on top of kegs. That said, I just can't see myself sacrificing Tri/IE/BT for it.
: I see a lot of higher level players picking this up, but I personally don't think it's that great. It's way too expensive for the stats it gives. That said, it is always an option to consider.
: Super situational. Might be a decent pickup for SOME measure of defense in a full damage build (and it would also let you stock kegs faster)
: Only get this if nobody on your team will man up and take it. Thankfully, someone will usually get it.
/
: Attack speed just isn't as good on GP as it once was (which is saying something) so I feel both these items are much more situational than they once were. Which one is better though? Well, with the advent of Powder Keg, I'd say Shiv loses most of the appeal it once had to good ol' GP, and I would argue PD is better on him. I occasionally take
instead of
in a 3 offense 2 defense build. You're trading consistent keg damage and survivability for random extra damage and dueling potential.
: Decent anti-burst option. Works well with
. I'd usually go with
or even
over this though. Also useful for pretending you're Mordekaiser.
tl;dr purchase defense goddammit