Piers Gaveston was made earl of Cornwall by the new king Edward II on 6 August 1307, and married Edward's niece Margaret de Clare on 1 November that year. (He would also be appointed regent of England on 26 December, to take effect when Edward travelled to France to marry Isabella.) 1307 was, in short, Piers Gaveston's year, and, being the great jouster and competitor he was, it's no surprise to find him holding a tournament to celebrate his good fortune. The tournament was held at his castle of Wallingford, a dozen miles from Oxford, on 2 December 1307. Edward II encouraged him to hold the tournament, though evidently he didn't attend himself, as his itinerary on that day places him firstly at Langley, forty-five miles away from Wallingford, then at Reading, twenty-five miles from Wallingford.  Edward had been at Langley since about 10 November, stayed at Reading for several days from 2 December, then travelled back to Langley via Bisham on the 6th. There is nothing to indicate a ride to Wallingford to watch Piers jousting, unfortunately.
The tournament was, however, attended by the earls of Surrey (Edward II's nephew-in-law John de Warenne), Hereford (Edward's brother-in-law Humphrey de Bohun) and Arundel (Edmund Fitzalan), and apparently other earls and magnates who are not named, as the Vita Edwardi Secundi (ed. Denholm-Young, p. 2) says that "there were ranged on one side three or four earls with a strong troop...and not a few barons." The Vita also says that "Sir Piers' side could not raise an earl, but almost all the younger and more athletic knights of the kingdom, whom persuasion or hope of reward could bring together, assisted him." The Annales Paulini (ed. Stubbs, pp. 258-9) accuse Piers, whether correctly or not I don't know, of fielding 200 knights instead of the agreed sixty, and the St Albans chronicler 'Trokelowe' (ed. Riley, p. 65) says that Piers and his knights "most vilely trod underfoot" the opposition. Oh dear, seems as though the high and mighty earls were defeated by Piers and his team and really didn't like it, and surely if Piers had tried to cheat by so dramatically increasing the number of knights on his side, the earls could simply have refused to compete. Here's the Vita, which incidentally doesn't confirm the story in the St Paul's annals and Trokelowe that Piers cheated in some way: "So it was in this tournament his [Piers'] party had the upper hand and carried off the spoils, although the other side remained in possession of the field. For it is a recognised rule of this game that he who loses most and is most frequently unhorsed, is adjudged the most valiant and the stronger."
The Annales Paulini say that Piers organised another tournament at Faversham to celebrate Edward's marriage to Isabella. There is no other information about this, and nothing that I know of to confirm that this tournament did indeed take place, though Edward and Isabella's route from Dover (where they arrived on 7 February 1308) to London (where they arrived on 21 February) would have taken them past or through Faversham, so it seems possible. The Annales also claim that this tournament caused anger among the barons, and that a third tournament which Edward II planned to hold at Stepney to celebrate his coronation on 25 February had to be cancelled when Piers told him he feared that the earls would have him killed if it went ahead and he participated. For the record, I don't know of any occasion when Edward II is known to have jousted. I wonder if his father forbade him from competing in his youth, given that the old king lost three sons in childhood and that for many years Edward of Caernarfon was his only male heir, and given the dangers of the sport. The earl of Surrey's son and heir William de Warenne was killed jousting in 1286 when Edward was only two, and Duke John I of Brabant, father-in-law of Edward's sister Margaret, in 1294.
The Vita Edwardi Secundi says that the tournament of Wallingford "roused the earls and barons to still greater hatred of Piers." Whether he broke the rules or not, the fact remained that he and his knights had destroyed the earls' dignity by knocking them off their horses into the mud, to their humiliation and anger. Not only did Piers Gaveston dominate Edward II's favour to an incredible degree, the earls could match him neither in wit nor in military prowess. Having said that, John de Warenne, earl of Surrey (whose father William was killed jousting in 1286), following a long period of hostility to Piers, changed his mind on Piers' return from his second exile in the summer of 1309: "Earl Warenne who, ever since the conclusion of the Wallingford tournament, had never shown Piers any welcome, became his inseparable friend and faithful helper." No wonder the author of the Vita, who records this, exclaims in exasperation "See how often and abruptly great men change their sides...The love of magnates is as a game of dice, and the desires of the rich like feathers." (pp. 7-8).
1) Elizabeth Hallam, The Itinerary of Edward II and His Household, 1307-1327, p. 26; Calendar of Patent Rolls 1307-1313, pp. 13-26; Calendar of Close Rolls 1307-1313, pp. 9-13.