When looking at abilities which double/halve stats, the difference is much bigger than you think. How would Azumarill, Medicham and Zweilous receive their reputations as hard hitters if the former 2's attacks are "doubled into 100 or 120 respectively" or the latter's "~127 attack stat"? To put into perspective, max Attack Adamant Gigas has roughly the same striking power as UNINVESTED PHYSICAL HEATMOR. Would you want to put up with something like that for 5 turns? As the nail in the coffin, Gigas doesn't have the proper means to stall out those turns. If it tries Parafusion, it loses out on coverage. If it tries Substitute, it's losing health that it can't recover easily. If it gets statused, it's as good as neutered/dead. I personally wouldn't bother with it.