The fundamental physics governing cosmic inflation remains unknown. A variety of proposed scenarios predict the existence of spectator fields during inflation. While such fields do not drive inflation, they can play important roles in the post-inflationary evolution. For example, they can give rise to an era of early matter domination (EMD) if they are cosmologically long-lived. I will show how the same spectator field can enhance the primordial fluctuations on small scales, give rise to an EMD era, and also source an observable stochastic gravitational wave background (SGWB) from second-order effects. The non-trivial changes in the equation of state in this scenario require a dedicated numerical computation of SGWB which we perform for the first time.