Battle Century G is essentially the 2nd edition of Giant Guardian Generation, a homebrew title meant to capture the spirit of Super Robot Wars, a popular video game franchise of tactical RPGs featuring mecha from just about every mecha IP owned by Bandai. If you've touched even a single SRW game, you'll feel very familiar with BCG, and many design decision will become very clear.
What the game offers is a satisfying tactical RPG that handles things like transformable mecha or combiners with ease. Everything his heavily flavored for mecha action, but with just a little bit of reskinning the system can be used for anything from Super Sentai to action anime.
There are plenty of ways to customize your mecha, but the general system is very streamlined - so much so that you get to name the mecha's areas aka hit locations, which is also a very clever way to ensure you can use the same system for just about any vehicle or monster.
Some might get put off by the fact you can create your own weapons, but the weapons are yet again very streamlined (usually falling into broad categories like "better in specific situations" or "better in general, but with downsides") and just begging to be reskinned.
The game also comes with a simplified combat system for out-of-mecha fights, but there's really nothing stopping you from using the main combat rules for more depth and customization.
[5 of 5 Stars!]