Oatmeal (if unsweetened/unflavored) has about the same GI as orange juice or cake made from some Betty Crocker boxed cake mixes and is just slightly lower-GI than the American formulation of Coca-Cola (see #333 Oatmeal (Canada), GI value of 54 ± 4 in the table at https://www.sciencedirect.com/science/article/pii/S000291652...).
I think oatmeal and other high-GI foods are promoted as low-GI in hopes of helping wean diabetics and pre-diabetics off of even worse foods (e.g. something really bad like powdered donuts), akin to how methadone is used to try to help heroin addicts. But genuine low-GI foods would be things like eggs, cheese, or chicken breast (all with nearly 0 GI) or some raw veggies like raw broccoli or the greens in a leafy salad (in the neighborhood, roughly, of 10-20 GI). Cooking low-GI vegetables like broccoli defeats the purpose and raises its GI to around the same value as oatmeal.
I think oatmeal and other high-GI foods are promoted as low-GI in hopes of helping wean diabetics and pre-diabetics off of even worse foods (e.g. something really bad like powdered donuts), akin to how methadone is used to try to help heroin addicts. But genuine low-GI foods would be things like eggs, cheese, or chicken breast (all with nearly 0 GI) or some raw veggies like raw broccoli or the greens in a leafy salad (in the neighborhood, roughly, of 10-20 GI). Cooking low-GI vegetables like broccoli defeats the purpose and raises its GI to around the same value as oatmeal.