Please note that Taeguk’s damage bonus is additive. That means it combines additively with all other additive damage bonuses, and that total amount is then applied as a multiplier to your weapon damage.
There are two types of damage bonuses in D3: additive and multiplicative.
Generally, multiplicative sources of damage (known as multipliers) are separate and do not add together; they multiply the sum total of the previous multiplier, hence why they’re so strong and valuable. And generally, the more additive bonuses available to a build, the less valuable other additive sources (such as Taeguk) will be.
Since Taeguk’s bonus is additive, it’s generally less valuable than, say, the multiplier offered by Bane of the Trapped. But certain builds, mostly those that channel their skills such as Zodiac Rend or Firebird’s, can greatly benefit from Taeguk’s bonus. What gems you use will depend on your build, so check the relevant build guide for specific details regarding gem selection.