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 (860)

Query time 0.00061

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "28.00"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "gp",
          "access_type": "ref",
          "possible_keys": [
            "idx_group_id"
          ],
          "key": "idx_group_id",
          "used_key_parts": [
            "group_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 40,
          "rows_produced_per_join": 40,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "10.00",
            "eval_cost": "4.00",
            "prefix_cost": "14.00",
            "data_read_per_join": "640"
          },
          "used_columns": [
            "product_id",
            "parent_product_id",
            "group_id"
          ]
        }
      },
      {
        "table": {
          "table_name": "p",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "product_id"
          ],
          "key_length": "3",
          "ref": [
            "test2_gaseus.gp.product_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 40,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "10.00",
            "eval_cost": "4.00",
            "prefix_cost": "28.00",
            "data_read_per_join": "261K"
          },
          "used_columns": [
            "product_id",
            "company_id"
          ]
        }
      }
    ]
  }
}

Result

product_id parent_product_id group_id company_id
12463 12424 860 1
12462 12424 860 1
12461 12424 860 1
12460 12424 860 1
12459 12424 860 1
12458 12424 860 1
12457 12424 860 1
12456 12424 860 1
12455 12424 860 1
12454 12424 860 1
12453 12424 860 1
12452 12424 860 1
12451 12424 860 1
12450 12424 860 1
12449 12424 860 1
12448 12424 860 1
12447 12424 860 1
12446 12424 860 1
12445 12424 860 1
12444 12424 860 1
12443 12424 860 1
12442 12424 860 1
12441 12424 860 1
12440 12424 860 1
12439 12424 860 1
12438 12424 860 1
12437 12424 860 1
12436 12424 860 1
12435 12424 860 1
12434 12424 860 1
12433 12424 860 1
12432 12424 860 1
12431 12424 860 1
12430 12424 860 1
12429 12424 860 1
12428 12424 860 1
12427 12424 860 1
12426 12424 860 1
12425 12424 860 1
12424 0 860 1