Gainz Box Coupon: Get $5 Off On Your First Box! Posted by Hello Subscription on April 19, 2019. Subscription Box Coupons & Deals / Gainz Box Coupons Hello Subscription may earn compensation via links in this post. Read the full disclosure Save for Later Notification Gain[z] Box has a new sale! Get $5 off your first box just use the code GZBX5. Valid to new subscribers only. Get $5 off at Gain[z] Box! Are you getting Gain[z] Box?