If you can't build a trap the best place to tame a Giga in the island map is to lure it into smugglers pass. After it kills everything it will, guaranteed get stuck in one of the cliff faces. Just keep tranqing it. Once it's down wait till it loses 4000 torpor and activate 100 narcotics. This gives you ample time to build wood/metal spike walls around the unconscious Giga.
I dare you to raise one!! They are savage if you got two high lvl parents (about lvl 140ish) and you get imprinting on it. The thing will become an adult with about 230% melee and basically one chomp any bronto!!! But on the down side you loose your job and your girlfriend
Make 4 metal dino gates and 1 large bear trap. Place three frames and door into an incomplete square and the large bear trap in the I between them. Lure the Giga into the trap and while its trapped place 4th down to make the square. Then just light it up with tranqs