For 5 mana, you can easily make a tough beets that can kill nurse gargantuan and live. You only need to have 4 hp on the tough beets, that's 3 other creatures, and one of them is the nurse gargan already. If you can't, 5 sun creature that trades for 7 brain creature plus heal 4 is more than worth it (2 brains heal 4? That's terrible value). This card is worse than other gargantuans because you don't need squash to kill it in order to gain value, there are so many options to kill this and gain value that generally the special ability doesn't even make up for it.
I wouldn't say it is unplayable, but it is a pretty weak card for 7 brains, I would play no more than 2x, and if anything I don't play this gargantuan as there are much better threats than this card.