You have the MacBookPro6,2, the Edsel of Macs. It may have a logic-board defect covered by this recall.
Make a "Genius" appointment at an Apple Store, or go to another authorized service provider, to have the machine tested. The routine hardware diagnostics used by service providers do not detect the fault. There is a specific test for this issue that Apple calls "VST" (for "Video Switching Test.") Ask for it. A "Failed" result means that the fault is present. However, according to reports, some units may pass the test, and yet still be deemed eligible for repair under the program. Your experience may vary.
Print the support page linked above, and bring it with you.
Note that the recall only applies within three years of purchase. After that, Apple may refuse the service. In that case, you may be quoted a price of about $300 (in the U.S.) for a "depot repair," which involves shipping the unit to a central repair facility and takes about two weeks. For that flat fee, anything found wrong with it should be fixed, not just the logic board.
The model was discontinued on February 24, 2011, so the recall has nominally ended. Some units may have been sold after that date by Apple as refurbished, or by resellers as new, and they should still be covered.
Some owners report that their unit was repaired under the program after the deadline had passed. Others report that the repair was covered by the extended warranty from a credit-card issuer. Again, your experience may vary.
Sometimes the replacement part is also defective, so be prepared for that possibility.
If it's too late for your unit to be repaired under the program, and you don't want to pay for a new logic board, you may be able to stop the panics by disabling automatic graphics switching.
If you're dissatisfied with Apple's response, please keep in mind that no one here represents the company or can help with customer-service issues.
Back up all data on the internal drive(s) before you hand over your computer to anyone. If privacy is a concern, erase the data partition(s) with the option to write zeros* (do this only if you have at least two complete, independent backups, and you know how to restore to an empty drive from any of them.) Don’t erase the recovery partition, if present.
*An SSD doesn't need to be zeroed.