I will say that you have the false info here. I will start by stating that every item that I mentioned as an offensive/semi-offensive option has a situational use on Gnar and that every one of the mentioned items has been used by professional players in competition as stand alone items or in conjunction with more offensive options. Check the games of Zion Spartan, Impact, and Smeb to see what I'm talking about.
Situational buys on offensive items early help you hold your own in lane and give you a damage boost in midgame teamfights even if you are going to be a main tank of the team. Hexdrinker is one of the best items into a magic damage opponent like Rumble to be able to effectively trade with him. The passive of Hexdrinker will also give a significant shield in the midgame to help you survive that extra few seconds to dish out a little extra damage or get away without giving a kill over to your enemies.
Another thing to consider is that if you have another main tank on your team (say a Jungler) and your other laners are behind or your comp is lacking in damage overall. Getting an offensive item or two can make you more of a threat to the enemy team and with the items of
, you aren't taking a massive hit to your overall tankiness, meaning you will still be a pretty good frontliner.
On Sterak's specifically, he is actually a fairly good user of the item right now and I think the item is only good on a handful of champs situationally, but Gnar can use it very well due to having a lot of Total AD scaling on his abilities, his AA's being a very important part of his damage output, and his Mega Form having the highest Base AD of any champion in the game (pretty sure) meaning he gets the most AD out of the item. His passive charges based on how much he outputs damage and how much he takes as well making Sterak's perfect for him. Gaining size and a shield (particularly in Mega Form) will also be very beneficial for his Crowd Control and tanking ability in Mega form.
On Black Cleaver, I was mentioning the preseason changes that are currently on the PBE for the item. The cost was increased to 3500 and the AD went up to 55 while the HP it gives has gone down to 300, so I felt like it was an overall nerf, at least to Gnar with the item as the cost increase hurts him and he didn't care about the AD as much as the CDR, HP, Rage passive and armor shred.
Lastly, I wasn't giving people advice like you stated. I was making an observation and stating my opinion on the matter. If people read it and thought on it, maybe they would come to the same conclusion or maybe they would land somewhere else. But I will leave with this. Whether or not someone plays ranked does not really indicate whether or not they have some basic knowledge of the game and can make assumptions based on experience and numbers of items, champions, and abilities about interactions within that game. Rank can be a very good indication on how much you know about the game as you have to know a really good volume of it to achieve higher ranks, but it is not a requirement of someone who has good game knowledge to have a high rank or to even play ranked at all. I personally have played over 2000 games of League. Maybe one day I would decide to try my hand at Ranked but I don't see the point other than to prove to a bunch of people on the boards that I have some idea of what I'm talking about.
Anyway. That's my rant. Later. Peace.