Nick Woltemade's inadvertent effort, turning Sandro Tonali's initial strike towards goal, fired the Magpies into a deserved lead at Lotto Park before Gordon's penalty doubled the visitors' advantage before the half-time break, becoming the first Newcastle player since Patrick Kluivert in the UEFA Cup in 2004/05 to score in their first two matches of a major European campaign.
Gordon extended United's tally further after the hour mark, netting once again from 12 yards, before Harvey Barnes wrapped up the scoring in the closing stages, completing a neat counter-attack, to help the Magpies record their first away win in a major European competition since 2013 in the Europa League.
Eddie Howe made two changes from Sunday's Premier League defeat against Arsenal as Kieran Trippier replaced the injured Tino Livramento while Anthony Elanga was handed a start in place of Jacob Murphy.
Newcastle survived an early attack from the hosts as Anouar Ait El Hadj's dangerous corner was nodded into Kevin Rodríguez's path but the Ecuadorian's close-range header was directed over the crossbar.
United, however, took the lead in the 17th minute as Woltemade, making his full Champions League bow, opened the scoring for the visitors. After Elanga's bending cross from the right flank was only cleared into Tonali's path, the Italian's resulting first-time effort was deflected into the net by the German striker.
Gordon went close to netting in successive Champions League outings as the England international latched onto Elanga's teasing cross towards the front post but directed a first-time effort, from a tight angle, over the crossbar.
Nick Pope, targeting a fifth clean sheet of the season across all competitions, produced a key save to prevent Adem Zorgane from equalising for the Belgian Pro League champions as a corner fell into the midfielder's path before his powerful drive from the edge of the box was pushed around the post by the 33-year-old shot-stopper.
Newcastle extended their lead in the 43rd minute as Gordon netted from the penalty spot. After the lively Elanga was fouled inside the box by Fedde Leysen, Gordon cooly dispatched his attempt from 12 yards after sending Union goalkeeper Kjell Scherpen the wrong way.
Elanga went close to netting his first Champions League goal since 2022 in the opening minutes of the restart as the Swedish winger collected Joelinton's pass inside the box before his low drilled shot at goal was pushed away by Scherpen.
United were fortunate to maintain their two-goal cushion shortly after as Anan Khalaili raced in behind the United defence and, despite holding off the challenge of Dan Burn, curled a low effort wide of the far left-hand post.
Pope produced two strong stops in succession, standing tall to block Ousseynou Niang's powerful drive at goal before quickly getting down low to hold Zorgane's strike from the edge of the box.
Newcastle netted a third goal in the 64th minute through Gordon's second penalty of the evening. After Gordon's inviting cross looked to have bounced onto Fedde Leysen's arm, the decision was initially waved away by referee Urs Schnyder before the intervention of VAR resulting in the awarding of the spot-kick, with Gordon confidently tucking the ball into the right-hand corner.
In the 80th minute, Barnes added gloss to the scoreline as the Magpies substitute netted his first goal of the 2025/26 campaign. After Gordon, Osula and Barnes all combined dangerously in the build-up, the latter was picked out by the Danish striker before placing a deflected first-time effort over Scherpen.
Tonali almost added a fifth for United in stoppage-time as the Italy international latched onto Lewis Hall's low pass before his fierce effort was pushed over the bar by Scherpen but the missed attempt would not prove costly as Newcastle sealed their biggest ever Champions League win on away soil.
Newcastle United: Nick Pope, Kieran Trippier (Fabian Schär 83), Sven Botman, Joelinton (Lewis Miley 59), Sandro Tonali, Anthony Gordon, Malick Thiaw, Anthony Elanga (Harvey Barnes 73), Nick Woltemade (William Osula 73), Dan Burn, Bruno Guimarães (Lewis Hall 73).
Subs not used: Max Thompson, Aaron Ramsdale, Emil Krafth, Jacob Murphy, Joe Willock, Alex Murphy.
