To be eligible for a gift card, you first need to have written your review through an offer where a gift card was advertised. Take note that not every review written will receive a gift card or was offered a gift card. If you did write a review through an incentivized link, and your review is approved, you will be able to find your gift card in the My Rewards tab of your G2 profile.
If you did not find the right gift card, feel free to open a support ticket and attach a screenshot of the original offer so we can look into it.