Ok I have a 00 gts and so does one of my friends. I have the same problems. I have do some research and here is what i found. I blew up my engine about 8 months ago and I got it covered under a goodwill warranty still costs me 2k but anyway I to the tech what was going on with my shifting, I had them install a fidanza flywheel and a ACT stage 2 clutch. They tore open my tranny while the engine was out, and my second gear was hashed. got it fixed and i still have the same problem, Toyota just doesn't make a smooth shifting tranny. Try either shifting slower don't bang it in 2nd or if you still want to shift fast hold the gas a little when shifting hard. it's the only cure i've found.
This is simply not true.
The problem is that the Celica was designed as a right hand driver, and the 2000-early 2001 tranny had a different shifter assembly/spring/etc that was wrong for the car, and it was updated. The reason the tranny doesn't hold up, is that the lever is pulling against the cable, because a right hand drive has a different pull on the cable.
I had my transmission updated to the new style after several shift linkages broke (4 total) and it's been great ever since. It used to grind going into 2nd gear, never went into reverse.
Now, it does all that smoothly. It's not that it's a poor tranny, it's got the wrong type of shift mechanism. Some people can use it properly, others can't. My linkage broke the first time, and since they no longer produce the old style linkage, toyota just stuck the new linkage on the old style mechanism, and it subsequently broke numerous times.
The updated transmission changes the gate position, shift spring, etc. Go read the TSB for the details.