Air Carrier Captain reported a fume event during gate preflight. After assisting maintenance with the engine run ups and maintenance checks which resulted in no improvements the flight was cancelled.

Narrative

Approaching the gate it was clear a serious fume event had/was taking place. Walking down the jet-bridge I almost immediately had a hard time breathing the air. Once I entered the cockpit; it was 100% clear it was fume event. I told the FAs (Flight Attendants) to deplane. Maintenance had NOT written up the issue much to my surprise. I immediately entered the fume event in the logbook. The air in the aircraft was so bad I had to depart to the gate area. Later I agreed to complete engine run ups and maintenance checks. The issue did not go away. Soon after I started to feel sick. I stated this to my FO (First Officer) and the FAs. Several hours later the flight cancelled and assumed I was going to the hotel. Scheduling demanded that I DH (Dead Head) back on a flight to ZZZ. I stated I needed a hotel room--I was told 'Sorry'. After hours of toxic air; maintenance run ups; and fume event reports; I was not feeling sick and fatigued but Scheduling would not grant my request for a hotel. I called out fatigued. I contacted [medical] after I called out due my stomach issues.