I had a newer style GT3100 that the deck was made very cheap and I actually bent the holes where the spindle mounted. I had to shim the spindle to get it to cut correctly.
You need one of these and a flat surface to park your machine on and measure side to side to make sure it is spot on. Then measure front to back and make sure the front is 1/8"-1/4" lower than the rear.
***all measurements are from the blade tip and check both sides of the blade.
https://www.amazon.com/Oregon-42-095...a&gad_source=1