[3] In 2018, Gaston won the Boys' Doubles at the Australian Open with fellow countryman Clément Tabur.

[4] The same year, he was named flag bearer for France at the Youth Summer Olympics in Buenos Aires.

Gaston also qualified for the ITF Junior Masters, where he reached the semifinals, losing to Tseng Chun-hsin.

In February, Gaston received a wildcard into the main draw of the 2018 Open 13 but lost in the first round to world no.

[6] Gaston received a wildcard into the 2018 French Open qualifying but lost in the first round to 24th Seed Jürgen Zopp 7–5, 4–6, 2–6.

[9] In April, Gaston won his first Futures tournament in Pula, Italy, defeating David Pichler in the final 6–4, 2–6, 6–3.

[7] His next tournament was in Lyon and buoyed by his French Open results, he reached the third round, beating Bernabé Zapata Miralles 6–1, 6–3 and world no.199 Tallon Griekspoor 2–6, 6–4 ret.

[8] His next tournament was in Tiburon where he equalled his best challenger result by reaching the third round, beating Evan Song and Daniel Galán before losing to eventual champion and world no.

[14] He continued his good run of form, winning Futures in Norman and Rodez defeating Michael Geerts and Benjamin Bonzi in the finals respectively.

[9] He played his last challenger of the year in Brest where he beat Constant Lestienne 7–6(7–3), 6–2 before losing to world no.106 Lloyd Harris 6–7(3–7), 5–7.

[17] At the end of January, Hugo received a wildcard into the main draw of the 2020 Australian Open in his first Grand Slam tournament.

In the third round, he played the top seed Lorenzo Giustino and beat him to reach his first Challenger quarterfinal.

This was the last tournament he played before the coronavirus pandemic halted professional men's tennis until August.

In September, he received another wildcard into the main draw of the 2020 French Open and was given an opportunity to turn his form around.

[24] Gaston made a breakthrough at the French Open, defeating countryman Maxime Janvier, Yoshihito Nishioka, and 2015 French Open champion and sixteenth-seeded Stan Wawrinka in five sets to advance to the fourth round of the tournament, where he exited in five sets to World No.

[27] Two weeks after the French Open, Gaston competed in Hamburg where he was forced to play in the qualifying draw.

In March, Gaston won his first match at the ATP masters 1000 level as a wildcard at the 2021 Miami Open defeating Dominik Koepfer.

In June at the 2021 French Open, Gaston reached the second round in doubles, on the 4th consecutive attempt being awarded a wildcard each time, partnering with fellow Frenchman Arthur Cazaux.

He received a wildcard for the qualifying competition in his home tournament, the 2022 Rolex Paris Masters,[39][40] but lost in the first round to Marc-Andrea Huesler.

As a result of not being able to defend his points from his quarterfinal run in Paris in 2021, he dropped by close to 50 positions in the rankings, out of the top 100 to No.

He received a wildcard for the main draw at the 2023 French Open where he lost in the first round to Alex Molcan.

In May, Gaston received a fine for intentionally dropping a ball during his match with Borna Ćorić in the second round of the 2023 Madrid Open, with the aim of forcing the replay of a point.

[43] In July, Gaston won his second title on the Challenger tour at the Iasi Open in Romania, defeating Bernabe Zapata Miralles in the final.

[44] In January, Gaston entered the 2024 Australian Open as a lucky loser and lost in the second round to 12th seed Taylor Fritz.

In June, Gaston won his first Challenger title of the season in Lyon as the top seed, defeating fellow countryman Alexandre Muller in the final.

[45] At the 2024 Generali Open Kitzbühel he reached the quarterfinals defeating Daniel Altmaier and seventh seed Roberto Carballés Baena.

[48] Thiem said of him "I haven't seen in a very long time a player with such a big touch in his hands, his dropshots are just from another planet".

[49] Gaston has a varied playing style and hits sliced forehands as well as backhands to destabilize his opponents.