Knocking out the giga is the hard part. After you accomplish this, drop kibble and biotoxin into the unconscious beast and sit atop its prone form. Keep the gigas inventory open and watch it's torpor plummet until it gets to around 1500-2000 then pump the biotoxin into it. As long as the gigas torpor is raising, it can't drop. For Knocking out a giga, spare no expense on a metal trap. Use behemoth gates and large bear traps. You can use normal metal gates but it's tricky and I have died and lost many tames this way. Another trick I have used is on The Island. Find a giga in the snow and lead it to the ice floes on the river. The giga will get stuck and then you can tranq it with ease.