I am not too sure what's G no Arashi about exactly, I've only watched a few episodes here and there. From what I've gathered from the few episodes, it's either they are playing games or doing experiments during the A no Arashi section. As for Mago Mago Arashi, two of the members will go to an elderly couple's place and be their grandsons for the day or something.
By the way, I think G no Arashi is going to have a revamp, the new show will be called Q no Arashi.
It's quite common for JE groups to have their own variety show, regardless whether they are amazingly popular or already pass their peak. Arashi has Mago Mago Arashi and G no Arashi, folks like KAT-TUN regularly appears on Utawara, SMAP has their own show SMAPXSMAP and so on.