A heavyweight T-shirt is often described with one number.
The buyer asks for a certain GSM. The supplier sends several fabric swatches. The heaviest option appears to be the most premium, so it becomes the starting point for the sample.
Then the T-shirt is worn.
The fabric feels substantial, but the neckline is too hard. The sleeves stand away from the arms. The body is wide and long rather than intentionally boxy. After washing, the hem twists slightly and the neck rib begins to ripple.
The fabric was heavy, but the product was not finished.
For brands searching for a custom heavyweight T-shirt manufacturer, the real objective should not be maximum weight. It should be a controlled relationship between fabric density, softness, fit, neckline construction, sewing and washing performance.
A premium heavyweight tee should feel substantial without becoming uncomfortable. It should hold its silhouette while still moving naturally with the body.