Both these sets of books are among my favourite fantasy. They're both grim and gritty, but the similarities end there.
The Gentleman Bastards books are in a lot of ways like the television series Hustle: slick cons where the reader is left unaware of parts of the plan until the end. Sadly for the protagonists, outside players come in and turn their carefully planned cons into bloody tales of betrayal and desparation. The setting for the first book is a blend of Renaissance Venice and Victorian London and more generic fantasy cities. The settings for the second book range from a similar Renaissance-like city to tropical seas. The writing is very accomplished, the characters are believable in what they can and cannot do and how they respond to each other, and the protagonists particularly are a nicely balanced between likeable and selfish.
The First Law series is much darker: an epic about the fate of the world, the countries in it and several individuals in those countries. The author is not afraid to show how unpleasant adventuring and wars can actually be for the people involved, and how futile much of it is in the grand scheme of things. The protagonists include a vain and selfish noble, a homicidal barbarian and a torturer, and the author succeeds in making them, if not exactly likeable, at least humans with whom the reader can identify. The descriptions of when "the Bloody Nine" emerges are some of the best descriptions of barbarian rage that I've read. Although I enjoyed the original First Law trilogy, I'd have to say that so far my favourite books in the series are Best Served Cold and Red Country.
In short, both sets of books are more of a modern, darker take on traditional fantasy. There are no quick and easy victories, lives are cheap and there's always someone more powerful ready to pull a fast one on the heroes. Think of the Belgariad and the Malorean by David Eddings, and then imagine pretty much the opposite.