I am going to first say that my opinion of this game might be more than just a tad biased as I have spoken with the creator of this game on numerous occasions via the game's dedicated Discord server. That said I will try to decouple any personal feelings and look at this game in as straight forward a fashion as I can. Also this will be a review for both Battle Century G as well as Battle Century Z.
Some background - I am a huge mecha nerd. Gundam, Escaflowne, Robotech, Evangelion and more (those are just the more well known ones I can think of at the moment). I have tried several systems and homebrew material for those systems including a clearly well-intentioned but sorely disappointing attempt to overhaul the third edition of the "World's Most Popular Roleplaying Game" some years ago. None of those ever worked. I've even looked at several systems that were dedicated to the mecha subgenre of science fiction and three "universal" systems. Either they proved far too skimpy on the rules or one would need multiple spreadsheets to handle just the base game.
None of that is needed with Battle Century G. The system is not rules heavy nor is it rules light. The overall system, rather, is effects based. See a really cool power in the book but don't like the default description? Take it anyway and refluff it as you see fit. Do you want a soldier in power armor taking down giant bioweapon monsters from the planet Xerixyxow 3? Do you want slow, lumbering behemoth mecha that attack just as readily with a rail cannon as try to hack the enemy mecha to shut them down before a single shot is fired? Do you want magical girls fighting arcane-infused demons from the shadow plane? Do you want to pierce the heavens with a drill while shouting manly phrases? All that and more is possible here.
The Books: Battle Century G and Z are clearly works of passion. While they are lovingly created and have tons of optional rules, guidelines and hints, I do find a few minor issues with the books. BCG's index at the back I found rather thin and, when coupled with the fact that most of the powers, weapons, etc. in the book are place in their respective sections they are rarely (if ever) in alphabetical order, something I find can (and does) slow down gameplay. Z is just as bad with failing to align things alphabetically. With a PDF I am sure there is little issue so long as you can have a search funtion for the PDF but with printed copies it can slow things down unless the game master or the players have written out the relevant information before hand. That said, each power is unique, each weapon fun and the character/mecha sheets straightforward and easy to read and use. One oddity I do find is that BCZ does not have a spine title and BCG does but it is upside down, reading from bottom up rather than the universal top down. Minor issue but one that I felt needed to be stated.
The Art: I'm not one to complain about art being a terrible artist in the real world so I will simply comment on a few key pieces I particularly like. The cover and back art splashes are evocative and commanding with BCG's clearly being influenced by the so-called "Real Robot" mecha subgenre; BCZ's cover is influenced by the "Super Robot" mecha subgenra with it's back cover that of a power armor wearing humanoid with a sword reminds me of certain anime/video game characters. The art within the books tend to be varied with some being of what I would consider really good quality and others not so much. One issue I found with my copy of BCG is that several of the images (particularly for enemies near the back of the book) are extremely faded. While that may be a printing issue, I will bring it to light that it is a thing.
The Actual Game System: Thanks to the extremely open-ended nature of the system, any form of mecha or high-fantasy/radical sci-fi game is possible. You won't be bogged down with multiple pages detailing how to make a robot leg, for instance, but will get instead multiple pages of easily refluffable Powers, Upgrades, Weapons and guidelines and tips on how to use them. Each of these are purchased with a resource called "Mecha Points" and as you play the game you earn more through combat and roleplaying. Each mecha starts with 100 MP and as one gains "Power Levels" they unlock more MP, 30 MP per Power Level. One does need to spend said MP right away it seems though it should be noted not doing to could leave you at a particular disadvantage against enemies. However, if you would rather have a "midseason upgrade" like we see in many mecha anime, I say more power to you (if you survive!). With the inclusion of BCZ, the list of options for Powers, Upgrades and Weapons expand all the more and by golly there are possibly millions of possible combinations by that point! I will say that while you may not need any spreadsheets one should probably have the book sticky-noted to key pages or have a couple of pages of notes to the side with all of the effects and such that the enemies and players can pull off to speed things up.
As for out-of-mecha goodness, it's there but not as well supported as I have seen in other games. That may or may not be to everyone's liking but I'm perfectly happy with it. After years and years of having characters with several pages of equipments and special abilites, spells/tech and more to handle, this is rather fresh change of pace for me.
Influences: It wears them on it's sleeve. Drawing from such disparate mecha series as Mazinger Z, Mobile Suit Gundam, Patlabor, Escaflowne, Big O, Macross and others, it wears it's lineage proudly, even calling out a number of these in text and with plenty of easter eggs for any fan of mecha anime and games to find (such as "Windmill with Arms and Legs" found on page 75 of BCG being a clear reference to the Nether Gundam from G Gundam, the "AIE-05 Angelus" on page 181 of BCG being a clear homage to the "Ramiel Angel" from Evangelion and the "Who the Hell do you Think I am?" power on page 15 of BCZ being something stated by Kamina in the series Tengen Toppa Gurren Lagann, just to name a few!).
Final Verdict: I never know whether I should go with a #/5 or #/10 system for these reviews. I think I'll stick with the latter but alas! This site uses a 5 star system. Oh well... Final Verdict for Battle Century G and Z is a high 9/10 or 5/5 stars for this site.
[5 of 5 Stars!]