Okay, first off, what kind of plugs were they?
If they were platinum, or any other conventional spark plug, they can be regapped. If they are Iridium, they come pre-gapped and should not be gapped before installation; this is because you can break the iridium tip.Instead, check the gap with a decent feeler gauge and, if the gap is off, take it back and get another plug.
next up, do you know what the gap is supposed to be at on your engine? You may be better off asking at a board that deals with your specific generation of celica... Ask in the classic celica forums. Goretro is a user on this board who runs a vintage celica, so he would know better than I.
You definitely should get a service manual for your car, that way, you will know exactly what your gap should be set at, as well as the heat rating for the plugs you need to run in your engine.
This brings me to my next point: Heat rating. Some plugs run hotter han others, use a plug that's too hot and you'll get detonation, use a plug that's too cold and you won't get a good enough spark. Make sure the heat rating is acceptable for the application. Sometimes, the guy at the parts counter will give you the wrong part for your application; it's happened to me a couple times
Lastly, How did you gap your plugs? did you use a round gapper, feeler gauge-type? wire type, or gapping pliers? Stay away from the round type, as they are tricky to get the proper gap with, and can become innacurate if dropped due to dings, warps in the gauge surface, etc. The wire-on-a-wheel ones are better, but can also become inacurate over time.
Feeler-type gapping sets are great, so long as you take care of them, but by far, a nice set of gapping pliers are the best way to do it. they are accurate and don't damage the electrodes.
So... the lesson here is: Make sure your plugs are the right type and heat rating, and use a reliable method for checking and adjusting the spark gap. It also helps if you have the manufacturers specs.
My guess would be that the gap is too wide. Pull the plugs and re-check the gap against the factory specs.if it's off, regap them and try again.
One last tip: don't overtorque those spark plugs! hand-tight is fine.