SELECT 
  product_id, 
  group_id 
FROM 
  cscart_product_variation_group_products 
WHERE 
  group_id IN (33, 98, 100, 51, 99, 101)

Query time 0.00027

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "28.41"
    },
    "table": {
      "table_name": "cscart_product_variation_group_products",
      "access_type": "range",
      "possible_keys": [
        "idx_group_id"
      ],
      "key": "idx_group_id",
      "used_key_parts": [
        "group_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 16,
      "rows_produced_per_join": 16,
      "filtered": "100.00",
      "index_condition": "(`cscart`.`cscart_product_variation_group_products`.`group_id` in (33,98,100,51,99,101))",
      "cost_info": {
        "read_cost": "25.21",
        "eval_cost": "3.20",
        "prefix_cost": "28.41",
        "data_read_per_join": "256"
      },
      "used_columns": [
        "product_id",
        "group_id"
      ]
    }
  }
}