Pretty much anything it wants to. Galactics are energy beings who have mastered the conversion of energy to matter and back again. Thus, when dealing with other races, they frequently take on an appearance that means something to that race.
Generally, they have found that looking like a gigantic version of the race perceiving them is the quickest way to get respect, so a manifested Galactic is generally at least ten (10) times bigger than the normal member of the subject population. In their natural state, a Galactic is a perceivable pattern of energy that fluxes and glows as it uses energy and absorbs it.
Galactics feed on energy. At the core of the galaxy, with its close-packed stars, systems in constant states of creation and destruction, multitude of black holes and quasars, and energy available everywhere, the Galactics thrive and maintain their energy shape. It is no place to try to enter uninvited.
In the Galactic Arms, a Galactic’s energy supply is more problematical. In the close vicinity of a star they are at their peak, but even in the so-called Life Zone of a solar system, where planets capable of nourishing humanoid life are found, a Galactic starts to feel as if he is starving. Some have gone mad and sought out other sources of energy. Some have learned how to live off of life essence and the galactic monsters known as Starbreaker and Galactus are born.
While essentially immaterial, Galactics have learned to be fascinated by material things. The Emperor is said to be surrounded by colossal creations of various material races. Sometimes, it is reported, he lounges on them or around them. Sometimes it amuses itself by tossing a no-longer-favored piece into the black hole, just to see (and otherwise sense) it being wrenched apart by the tidal strains. Other Galactics like to maintain material body shapes and use such constructs as satellite-sized space ships and energy channeling weapons. But every Galactic has the ability to go energy-form again and use its powers without the need of props.
It is easy to think of the Galactics as gods. In many ways they effectively are. But there are other powers in the Universe that did not get their abilities from Galactic-spawned viruses. If one assumes there is a Creator, then the Galactics are amongst His creations, just as the smallest insect on the smallest planetoid. And there are certainly other gods out there, including Cthuluvian monstrosities from outside of normal space and beings who developed as powerful in their own way as the Galactics have.
Many of these are magical entities, personifications of the life force of the Galaxy. Where the Galactics are strong in the core of the galaxy, these beings do better where energy fosters life, rather than more energy. Such forces as Gaea and other planetary life forces belong in this category. Relative power is difficult to measure between such entities, but it is a measure of the power of the Life Force that it creates life, and the best the Galactics can do is manipulate life.