can i change my toshiba satellite p850 into a gaming laptop?

The max upgrades that you can get your laptop for it to be considered as a "GAMING" laptop, is by upgrading its base components with a GeForce GT 630M 2GB GPU, 16GB DDR3 RAM, Core i7-3610QM 2.3GHz Processor.

Ask the nearest Toshiba store if they offer that kind of upgrades for your laptop.
 
When you improve you laptop you are also giving heat that there was not there before,
You may be luck if the pc can dissipate the extra heat or not...