1999 Pontiac Grand Prix head gasket

Engine Performance problem
1999 Pontiac Grand Prix 6 cyl Two Wheel Drive Automatic 219k miles

I was told I have a head gasket problem. (Car overheats no apparent reason all obvious signs are fine & car runs well except a little sluggish between 60-70 mph). Is there any way to find out if it is just a gasket or will I need to replace my engine? Everyone I talk to seems to tell me I can't just replace the gasket, if it's the heads then I can't just replace the "top end". I'm very confused, and not in the position to afford a monthly car payment. Please help me understand the whole head gasket scenarios. Thank you!