Belle found Gaston in the Underworld and stamped a deal with Hades in "Once Upon A Time" season 5 episode 17.

"Once Upon A Time" season 5 episode 17 is entitled "Her Handsome Hero." According to Spoilers Guide, Belle saw Gaston in the Underworld, which prompted her to make a decision that can be a threat to her Rumplestiltskin. As per the publication, Belle asked help to the latter, since she was hopeful of finding her way to defend her and Hades child.

While there was some negotiations between the two, it was revealed that Belle and Rumpelestiltskin disagreed to use magic, with the former forbidding the latter to use his powers for a bad cause.

Episode 17 also saw Hades meet Gaston exclusively in the Underworld, telling the latter to seek vengeance on Rumplestiltskin. The episode saw the heroes as they attempt to unlock the path to defeating Hades, with Emma having a horrible nightmare that she thought was starting to actualize.

The episode also showed a flashback, featuring Gaston and Belle's first meeting. Further sequences revealed that the brewing romance between the two was disturbed on the eve of the wars between Ogres.

According to TV Line, the events in episode 17 were tragic, since Belle and Rumplestiltskin thought that they stick to their agreement with Hades. However, the god of the Underworld relished that the details of the deal was to throw Gaston in and not Belle.

Episode 18 is entitled "Ruby Slippers." As per Spoilers Guide, the episode features a flashback that sees Ruby and Mulan in Oz, as they are set to meet Dorothy. As per the publication, the trio see Zelena's return to the kingdom, which leads them to think of a way to defeat the latter. Dorothy disappears, so Ruby works hand in hand with Emma, Snow, and Regina to look for her. 

"Once Upon A Time" season 5 episode 17, "Her Handsome Hero," aired on Apr. 10, while episode 18, "Ruby Slippers," airs on Apr. 17.