gastar vs pasar (spend)
Both translate 'to spend' in English — gastar is spending money, pasar is spending time.
gastar
To spend money, or to use up / waste something.
Gastó todo el sueldo en ropa.
She spent her whole salary on clothes.
Money being spent. Pasar el sueldo would mean 'pass the salary' — nonsense.
No gastes tanta agua.
Don't waste so much water.
Gastar = to use up a resource. Wider than just money.
¿Cuánto has gastado?
How much have you spent?
The natural question about expenditure.
pasar
To spend time, to happen, to pass — one of the most versatile verbs in Spanish.
Pasamos el verano en Galicia.
We spent the summer in Galicia.
Time, not money. Gastamos el verano would be wrong.
¿Qué pasó?
What happened?
To happen — one of pasar's most common uses. Completely different from spending.
Pásalo bien.
Have a good time.
Pasarlo bien/mal = to have a good/bad time. Fixed idiom.
Tricky bit
English 'spend' covers both. Spanish splits them cleanly. The error 'gasté tres horas esperando' (I spent three hours waiting) sounds like you paid for the waiting. Use 'pasé tres horas esperando' or 'estuve tres horas esperando.'