First of all, IG maker and GG maker are not part of the RPGMaker product line. While RPGMaker focuses solely on JRPGs, IG Maker and GG Maker are less genre-focused game engines. I don't know why they list them on rpgmakerweb.com. Maybe because they can also make RPGs. (they also present MangaMaker on the website which isn't even a game development tool). The same company also currently develops VN Maker which is an engine especially for visual novels.
RPGMaker is a product which exists for quite a long time and went through several versions:
- RPG Maker 95
- RPG Maker 2000
- RPG Maker 2003
- RPG Maker XP (2005)
- RPG Maker VX (2008)
- RPG Maker VX Ace (2011)
- RPG Maker MV (2015)
The versions are only partially compatible with each other, which means a game developed on an older version can often not easily be ported to a more recent one. That means older versions still stay relevant for maintaining the older titles, for people who simply dislike some feature-changes of the newer versions or for people who want to benefit of the more mature ecosystem of previous versions (3rd party plugins and assets).
A complete list of all differences between all these versions would be too broad to list here. But when you are starting a greenfield project you should use the most recent version (RPG Maker MV).
GG Maker or IG Maker might be interesting when you plan a project which is sort of JRPGish but defies many common UI conventions of the genre. But then you should also consider all the other game engines out there and also the option of not using any game engine and just a bunch of helpful libraries for your programming language of choice.