Frozen Mallet on Gnar

I have seen on Imaqtpie's stream where he played Gnar toplane and bought the Frozen Mallet first. It gives a nice amount of tankiness too, but the item is for more situational purpose. If your team lacks crowd control I would pick that item up. Frozen Mallet is also a more succesful way to secure a kill when your jungler comes to gank and vice versa, to escape more easily when the enemy jungler ganks. Since the slow also applies on ranged attacks, I think it's an excellent item for Gnar (I would recommend not building Trinity Force if you choose to go for Frozen Mallet).

Well, first and foremost you need to realize Gnar is a tank. That means you shouldn't always rush a TF. (It's a great item on him but you need to be able to fill your role in a team.) If you get behind you should get tanky first. Sometimes you might want to just build the early phage for kiting/harassing purposes and need to get tanky. I would only finish TF early if you are way ahead in lane. The style of play you do with the TF is correct though. A TF build on Gnar is great for split pushing, but you need to be able to be useful to your team if the enemy tries to group and shove down a lane.

Now, that out of the way I'll get on to your question. Frozen Mallet is actually really good for split pushing even with a TF. It adds damage and it gives the enemy less counterplay if they try to stop you from pushing. I would definitely get some Resistances and other defensive items before getting Mallet though. I would build at least a Randuins and Banshees before the Mallet. They both give you high resistances and add to your base HP as well. With decent resistances it's not bad to stack on more health. Again, keep in mind that Frozen Mallet is Circumstantial. If you are trying to solo siege a tower with shorter trades a Warmogs might actually be better since it will be passively healing you up. If you are team fight oriented, I used to say always get Warmogs over Mallet since it was a 300 HP difference but the current patch brought it to only a 100 HP difference. In a team fight you only need to worry about burst and so a Frozen Mallet is almost as good as Warmogs in this scenario.

If you do manage to get far ahead and get that early TF, a Frozen Mallet might be a good thing then. It will allow you to VERY HEAVILY abuse the enemy. The kiting ability and damage should allow you to easily 2v1 pretty much anyone at this point also. After this you will just need to build your standard Randuins/Banshees to get some resistances.

Also, as an additional note, if you are going Brutalizer it's a little bit harder to kite enemies (although not too bad since gnar is naturally good at it) but Frozen Mallet can make it easier to do while giving him some tankiness as well.

TL;DR; Gnar is honestly the only champion I would agree with a Frozen Mallet. Don't get it every game though. Look at the game and analyze what you are lacking.