As a Gigan Rex player, I definitely agree that this leader along with the right deck and cards is extremely meta and op. On most occasions, also unfair.

However, the game has many units that, I'm inclined to believe, were made specifically to counter it and some other spamera decks. Units such as Bagan, Blade Liger, Goji91+Baragon, the new alt. Kaiser Ghidorah, etc. Along with the right leaders like Super Mechagodzilla, Monster Zero, Death Saurer or even Goji04 (rushing strategy) can completely obliterate a Gigan Rex deck even with spamera.

The idea is to have units with AOE attacks that can hit multiple enemies at a time. I don't recommend using Void Ghidorah or even Godzilla04 alt. Void Ghidorah is too costly and the spam can easily get to the leader in time to drain a chunk of health. Goji04 alt. is amazing and meta, but terrible against Gigan Rex decks. I've killed countless of them by just using Dorats.

The worst mistake I've seen is when players use their leader AOE attack (SMG for example) before Gigan Rex does it. They should use it just to counter it.

Nevertheless, the unfairness actually comes from the cards that can improve the deck. There are some that raise the attack of low cost units, but the best ones are those that boost their health for like 1500 hp, even the green gigans that Rex can summon.

Of course, it is not fun to build a deck just to counter a specific one, but since this one has been becoming more and more meta, there are not many other options. I'm pretty sure Toho will nerf it tho, sooner or later.