Alert rule that fires when the count value/output of a query changes

I have a widget that shows with the number of gateways that haven't been seen (not been online) for >= 2 days (The output is basically the most recent refreshed date and the value, aka the count of hubs not seen, as two columns).

I want to set up an alert rule that will notify me if that count number changes. E.g. current count is 2 (2 gateways haven't been seen for >= 2 days) and now it changes to 1 (e.g. because on gateway has come back online, so only one hub hasn't been seen for >=2 days) and that change I want to be notified about (and also in the other direction, when more gateways are added to the count as they haven't been seen for >= 2 days).

CREATE TABLE gateway_status (
    gateway_id VARCHAR(50),
    last_seen TIMESTAMP,
    status VARCHAR(20)
);

INSERT INTO gateway_status VALUES 
('gateway_001', '2024-06-09 10:00:00', 'offline'),
('gateway_002', '2024-06-09 10:00:00', 'offline'), 
('gateway_003', '2024-06-11 09:00:00', 'online');

--Current Query
sqlSELECT 
    NOW() as "time",
    COUNT(*) as "value"
FROM gateway_status 
WHERE last_seen <= NOW() - INTERVAL '2 days';

Desired alert behavior:

• Count changes from 2 → 1: Alert fired

• Count changes from 1 → 2: Alert fired

• Count changes from 0 → 1: Alert fired

• Count stays at 2 → 2: No alert

How best to approach this?

I tried a lot with ChatGPT which always suggests adding a new query and using diff() function, however the diff option doesn't show up for me. I know how to set it up so it alerts me when it becomes more than 2 but I can't figure out how to set it up so it also alerts it when it changes in the other direction.