To put it differently: To observe something it must interact with some meausurement device or directly with your body to perceive some signal with your senses, but this means it must exchange energy with the measurement device, and if it has no energy there's nothing it can exchange and thus it simply doesn't exist.
Electromagnetic waves, and light is nothing else than an electromagnetic wave with frequencies we can sense with our eyes, of course carry energy, momentum, and angular momentum and thus it can interact with measurement devices or our retina to sense it.
Now the Einstein field equations basically say that the sources of the gravitational field (described by the Einstein Tensor related to the curvature of spacetime) are all kinds of energy, momentum, and stress (described by the corresponding gauge-invariant symmetric energy-momentum tensor of the corresponding fields), and this implies that also electromagnetic waves are contributing to these sources and thus cause a gravitational field.