Are CPU and GPU embedded on the logic board?
As you can see on the image below, the CPU (red border) and the GPU (orange and yellow border) are wave soldiered onto the logic board and they will be replaced together.

BUT, you should know this kind of replacements are extremely expensive. Prices goes up to $1,000 so in most cases it's better to buy a new Mac. It's not uncommon to find "older" refurbished Macs at similar prices to logic board replacements.

You Mac is almost 3 years old and if I were you, I wouldn't do the repair and just go for a new Mac.