Do all IgG autoantibodies lead to autoimmune diseases?

As usual with biology, it's not that simple. There are autoantibodies present even in people without any signs of autoimmune disease. The immune system is a complex network of positive and negative regulatory cells and molecules - looking at a single component is unlikely to determine the state of the system. This is a major reason that it has been so difficult to figure out how to selectively act on the immune system to stop or prevent an autoimmune disease without compromising normal immune function.

Here's an article that expands on some of these aspects: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2703183/

For example, note "Despite the physiological elimination (negative selection) or functional inactivation (anergy) of high-affinity, self-reactive T and B lymphocytes in the thymus and bone marrow, respectively, there is compelling evidence that low-affinity, potentially autoreactive cells persist and that low-affinity reactivity to self-antigens is required for survival of T and probably B lymphocytes in the peripheral immune system."

It's also important to keep in mind that affinity of Abs and receptors for antigens usually depends on additional 3rd party molecules involved in the interaction, and that a single molecule may present very differently in different situations (for example, damage or inflammation may alter some self molecules into a previously unseen configuration that stimulates self-reactivity).