Loki/Gabriel has all the abilities of an archangel of the Lord along with abilities he’s developed to appear to be a pagan god. His abilities are fueled by angelic grace which is innate and inexhaustible.
In his first appearance he demonstrates the ability to physically manifest anything he wants - from candy bars to a convincing Incredible Hulk. His illusions can be applied to himself and Loki can take on any appearance he wishes. He can affect the material world from a distance as demonstrated by making Sam’s laptop disappear and letting all the air out of the tires of Dean’s car. He sits back and munches on candy and popcorn, laughing while Dean and Sam get beaten up by porn stars he created. Loki fakes his death by creating an illusion of himself while he remains invisible, watching the action.
The next time the trickster appears he manipulates time, creating a time loop of a single day that revolves around Sam. He later turns back time a full six months, only Sam being aware of the change in the world. The ability to manipulate time like this is something that the mist of Anatole will have diminished as to be non-existent, something that even Loki won’t realize he’s lost until there comes a situation when he attempts to do it.
When Loki/Gabriel/The Trickster next appears it is clear that his illusions can affect entire areas and are as physically real as anything natural occurring. He creates an entire pocket dimension inside an abandoned warehouse filled with human-seeming illusory characters. He turns Sam into a car with a voice ala Kitt in Knight Rider. He traps Castiel in what seems to be a pocket dimension from which he escapes, obviously wounded in the effort. Like all holy being, Loki/Gabriel can be trapped by holy oil, much as a demonic being can be trapped by a salt line or devil’s trap.
The most information we learn about Loki/Gabriel is in the episode “Hammer of the Gods.” Angels have an ability to “fly” that is similar to teleportation, usually accompanied by the rustling sound of wings. When Loki makes sudden entrances and exits he makes no such sounds. It’s unclear if he’s learned how to silence the effect or if he’s learned another method of instant travel. He possesses an archangel’s sword with ahs the ability to kill both angels and demons. When Dean attempts to say his name in fron of the assembled gods, Loki silences him by cutting off his voice. He creates a DVD out of thin air with a recording of himself on it and appears on it with a porn star he manifested hanging all over him.
There are many abilities that the angels and archangels in the Supernatural verse have been demonstrated to have, which while Loki does not display use of, would also have these abilities. Some of these would include the ability to communicate with other beings of a heavenly origin telepathically, instantaneous healing (of both himself and others), telekinesis, and the exorcism and/or smiting of demonic beings. He could destroy or repair any physical object or make it appear or disappear entirely.
http://en.wikipedia.org/wiki/Characters_of_Supernatural#Angels Normal humans cannot look upon or listen to an angel’s true form - a massive being of light and energy and grace - without burning out their eyes, ears and eventually dieing of overload. As such they take on a human form by possessing a human vessel with their permission. It’s not clear if Loki’s form used to a human or if it a construct he made himself. He makes a reference to getting a face change after he flees heaven, but Castiel still recognizes him for who he is behind that face.
He would also have the ability to sense the basic nature of a person’s being - for example he would identify all shinigami to be some sort of reaper. Loki would also have an extensive knowledge of languages, history, and rituals such as creating wards against different kinds of beings. He’s also shown the ability to hear conversations from long distances - even through walls - and is capable of some level of mind reading.
To sum up, as a being of essentially pure energy that is assuming a human form, Loki has the power to manipulate the reality around him. He’s a master of illusions and creating things out of thin air. While this usually takes the form of tricks, pranks and illusions, he could put it towards more useful purposes with the right motivation.