A giga is really hard to tame, but with the proper preparation and patience you can do it.

First, stock up on narcotics and either a tek bow if you can or if not just a good longneck and a ton of darts.

Second, find a giga and make a trap near it. You can get 4 behemoth gateways and doors and make a square and leave the front and back door open. And in the middle put a couple bear trap to keep the giga caged.

Finally, use a fast flyer with a lot of health and stamina (probably a griffin or an pterodactyl) and lead the giga in, quickly close both doors (prepare to be hit so bring extra armor and healing) and just shoot away, put it the food, wait for it to tame feeding it narcotics and ta da. You have a giga.