decoder.elm

import Json.Decode exposing (Decoder, field, string)

gifDecoder : Decoder String
gifDecoder =
  field "data" (field "image_url" string)

response.json

{
  "data": {
    "type": "gif",
    "id": "l2JhxfHWMBWuDMIpi",
    "title": "cat love GIF by The Secret Life Of Pets",
    "image_url": "https://media1.giphy.com/media/l2JhxfHWMBWuDMIpi/giphy.gif",
    "caption": ""
  },
  "meta": {
    "status": 200,
    "msg": "OK",
    "response_id": "5b105e44316d3571456c18b3"
  }
}