SELECT 
  gp.*, 
  p.company_id 
FROM 
  cscart_product_variation_group_products AS gp 
  LEFT JOIN cscart_products AS p ON gp.product_id = p.product_id 
WHERE 
  gp.group_id IN (31)

Query time 0.00031

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "gp",
      "access_type": "ref",
      "possible_keys": ["idx_group_id"],
      "key": "idx_group_id",
      "key_length": "3",
      "used_key_parts": ["group_id"],
      "ref": ["const"],
      "rows": 3,
      "filtered": 100
    },
    "table": {
      "table_name": "p",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "ref": ["cscart_purchase_fl_com.gp.product_id"],
      "rows": 1,
      "filtered": 100
    }
  }
}