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 (10)

Query time 0.00035

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "gp",
      "access_type": "ALL",
      "possible_keys": ["idx_group_id"],
      "rows": 7,
      "filtered": 100,
      "attached_condition": "gp.group_id = 10"
    },
    "table": {
      "table_name": "p",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "ref": ["c.gp.product_id"],
      "rows": 1,
      "filtered": 100
    }
  }
}