If the Creator or someone who wishes to mod this game ever get to updating it. I hope they add a path where you get Gabriela as a partner after you beat her. With the storyline that after she got raped by her in the attempt to get Max on your side, Amy becomes more submissive to her with extra scenes to develop a storyline between them. With one of the possible ending becoming her slave/submissive similar to how Maxine was when you first encounter them but with Amy being happy in that position. With the opportunity to become the dominant one in the relationship after you beat or or remain submissive if you so desire resulting in two different endings.
As a lead in for including her in the story, if you are to include her after beating her and Max you can add the option when you are prompted on whether or not you want to switch your current partner for Max, or shortly thereafter similar to how Eve was introduced. With her motive being to win the Championship and seeing you as superior to Max after beating the two of them and just needing the right "partner" to beat the best of the best. (This would require you to rewrite the scene where she decides to leave instead of being an option after you beat her. With this in mind an option to open up 2 different paths (should you decide to take her as a partner) is to state how Gabriela needs Amy and not the other way around (Dominants Route) which would open an either neutral or more dominant role in the relationship. With the second path, being an acceptance of what she says with her "rewarding" you for accepting your new role and relationship with her. This would bypass any argument of her being the clearly weaker choice between Gabriela and Max and provide a new story route for those who choose it.
If you want to create a method for introducing the relationship prior to the match, you can make it so after you have intervened in Gabriela and Max's "training" at the beach and volunteered to take Max's place. You leave with the option of either secretly liking the rough treatment or not. With the option of liking it leading to a more submissive route and not liking it leading to a revenge /dominant route. In the following days, providing you have been dating Max, Gabriela will confront Amy due to Max's lack of focus and newfound resistance to her. Leading to more scenes in which she harasses Amy, resulting in you obtaining Gabriela's number either due to having a newfound relationship as her bitch or by happenstance after resisting in further meetings. (the method is up to you) This submissive route giving you a minor debuff during your match and a dominant route giving you a minor buff during your match with Gabriela and Max. With an option, to confess your love to Gabriela after several meetings/dates and an interesting night if you completed all your dates with Max and Gabriela before meeting Max at night. With the option of contacting Gabriela before heading home to encounter Max there with Gabriela, wherein you both become Gabriela's Pets for the night and the option to further solidify the relationship with the clear understanding that the upcoming match will be all out. Leading to a harder match but more intimate relationship with the two of them regardless. As well as the option to meet Gabriela for another meeting at home during the night where in you further your dominant role or completely give into her and agree to be her pet (potentially with her saying you should throw the match for her) at which point you can agree. (You will play the match as normal) Upon victory, she is shocked and wants you to be her new partner into the championships allowing for you to change partners. Upon Defeat, a spicy ending with a small epilogue of how she broke any resistance in you towards her and eventually replaced Max with you, granting you another match set the following year where you and Gabriela can continue in a Championship match without Gwen Du Lac as an opponent. Where Victory and Defeat have different scenes. relating to where your relationship goes as her pet/partner given the time she invested in making you her own.