What is the problem when gauges are erratic up and down? on 2004 Chevrolet Trailblazer

I got a new battery and checked alternator

Asked by for the 2004 Chevrolet Trailblazer
2 answers
Ihad an alternator installed but now the check gauge come on and and the battery gauges drop all the way down whwat you yhink the problem is?my truck is a 2002 dodge dakota 3.9 engine
maybe there is an electrical issue