For attribute queries, the documentation says: "For numeric attributes, we can use the operators <, <=, >, >=, and == to specify constraints on their values, so a constraint on the attribute price looks like this: [price <= 150.0]". This however does not seem to work any longer, for quite a while now. You can check this on the API demo page - even Google's own sample query "snippets?bq=digital camera [megapixel >= 3.0]" fails with error 400 Bad Request. The only operator that works is ":".