[OkDually]

I had boughten a 1997 GMC Sierra K1500 with a 6.5 Turbo Diesel. I had completely removed the egr valve and changed the center head to be a delete but still bringing up the code. Do I have to change the ECU if so could I just flash it myself instead?

Welcome

The ECM will need a tune programmed into it. to remove the code.
If you have the programming equipment you could do it yourself.
Otherwise, you'll need to send the ECM into a vendor like Kennedy Diesel to get it programmed

 

[OkDually]

I know mechanical work not computer work and the truck shouldn't had the egr that was on it from the beginning.

Those were Standard installs from the factory on the 1500 series trucks