Why is G‑d always referred to as “He’, isn’t G‑d genderless?

G‑d is neither male nor female.

The word for "He" in Biblical Hebrew can also mean "it".

Put somewhat differently: when you go into a hardware store you will find two types of pipe fittings, these are described as "male" and "female". Pipes are obviously neither male nor female, but they are defined by their design of either being on the giving or receiving end.

So on a deeper level, all of G‑d’s names/titles are allegorical descriptions of G‑dly manifestations and interactions with us. In this sense we can say there are "masculine" and "feminine" expressions of G‑d.

In the relationship between man and woman, man provides the seed, and the woman develops the seed and produces a child. Similarly, we refer to G‑d as "He" because He supplied the world and all its contents, but expects of us – the allegorical female - to develop all of creation, to make it a holy place where G‑d's presence can be revealed.

Coincidentally, there is also a feminine aspect/name of G‑d: Shechinah. Shechinah represents G‑dly attributes that receive; either from other G‑dly attributes, or from our deeds.