erm, not to discredit YOU or anything, but since i'm being dissed here i'm going to have to agree by saying: "as i've said countless times before, it's there for a reason; balance"
truant gives makes you move every 2 rounds, there wasn't really a point other than slowing the monster down, it would've been better off with any other ability, even useless (in battle) ones such stench/illuminate/run away
study gigas's moveset, i can't find anything that "benefits" perse from a lower attack or speed...
i've theorised that switching gigas in during trick room would result in gigas moving first for as long as trick room is still in effect (and his own ability stil in effect, as half of his speed is well below the average speed, but there are alot more pokes who are slower still)
where as the halved attack... honestly, other than the fact that if a swagger or confuse ray hits gigas, it won't damage itself as much as it would've had it had its full monsterous attack strength, which would end in its demise rather quickly... but that is just an external effect, the side effect of having low attack, not literali any real synergy
most of this monster's moves are attacks, there doesn't seem to be much synergy...
hmmm but i do encourage you to keep studying the bulky thing, maybe you'll find something even i cannot? honestli, i onli just skimmed thru its list of moves...
(i KNOW this thing is a monster in double battles though, just not single)