Here's one for GT2 (version 1.0, the buggy one). You can't look at your garage if you're using this code, it will freeze the game.
Just save the game after you are awarded the prize car and license, or you'll have to start over.
8002F810 0000
80046E84 0000
It's probably easier to load the codes for all-gold licenses, and use the "Buy Any Car at Toyota" method to choose your prize cars
50003CA4 0000
801CA758 0400
* Do not load this code with your memory card, turn off at "warning", turn back on after loaded (if using old GS)
D01C322C D5D8
801C3018 D758 \
D01C322C D5D8 |- Spoon S2000
801C301A 120D /
D01C322C D5D8
801C2FCC A498 \
D01C322C D5D8 |- Concept Car
801C2FCE 0E35 /
D01C322C D5D8
801C3064 E59C \
D01C322C D5D8 |- 3000 GT LM Edition
801C3066 1745 /
D01C322C D5D8
801C30B0 F71C \
D01C322C D5D8 |- del Sol LM Edition
801C30B2 1200 /
D01C322C D5D8
801C30FC E65C \
D01C322C D5D8 |- FTO LM Edition
801C30FE 1701 /
D01C322C D5D8
801C3148 129C \
D01C322C D5D8 |- GT-One'99
801C314A 1E0D /
I've never found these codes to freeze up the game. Have at least 5.5 million credits to buy these cars. To play it safe, buy the cars, check your trophy room, and then save the game....and turn off the codes to play the game for real.
(BTW, I didn't create these codes, I just copied them from various sources. Do a search for more.)