Will my iMac run Garrysmod or Minecraft without any lag? or hardly any?

Update: listen i know Macs aren't exactly the best option for gaming but i have no other choice :( i just wanna know if i can get a buttery smooth 60FPS. system specs:
3.2GHz quadcore intel core i5 with 6MB L3 cache
8GB of 1600MHz DDR3 SDRAM
1TB 7200-RPM hard drive
cheers!!
Update 2: also!! its apples most recent iMac to date 27' display
Update 3: its a brand-new iMac btw. Un used completely
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