Coolant and transmission

I just purchased an 03 impala and i checked my radiator i have a build up of gunk in it i believe it is from using the wrong antifreeze but i am not sure anyway there also seems to be a leak from the head gasket on the forward head is there a default with that engine in these terms or should i just flush the radiatr and see what happens and can i change the transmission fluid myself or do i have to take it to a shop to change it?

I'd start here with what engine?

3.4 sorry

Get this checked more closely it is more likely the intake gasket is leaking and the cooling system will need a thorough flush. The trans pan comes down, the fluid is drained, filter changed, about 7 qts of Dexron 111, messy job.

thanks for the info i will deffinitly do that