what happeneds when i drive my car without a gas cap????

Nothing. There is a little cover that won't allow gas to come out.

I had mine stolen once. I just got a replacement the next day. No harm done.

Some fuel may evaporate out, some dirt or water may get in, SES light might come on.

EDIT: That fuel flap, as I like to call it, will prevent most evaporation or debris, but not all.

my dad claims his 98 chevy truck won't run right unless the cap is screwed on till it clicks.

Maybe because the SES light comes on with OBD-II equipped vehicles.

OBD I vehicles do not monitor tank vacume (unlike OBD II). It won't set off the check engine light on your car. As far as your dad'* truck...it will run but it will eventually set off the engine light. It is only emmisions (hydrocarbons) that it is monitoring and this has nothing to do with the running of the vehicle. Ask him to show you....hehe.