Atletico Madrid are far by 7 points from Girona and a victory could reduce the gap but essentially keep them away from Athletic Bilbao just behind them, so they wouldn't drop from Champions League spots.
For Girona, every victory done will make them more secure to play Champions League next season.
Odds are better for Atletico Madrid and personally I go with a draw.
Both teams are currently in the top 4 or the Champions League zone next season, but I think in this case both teams still need a win to secure their position, so between the two it is clear that Atletico Madrid has a heavier burden compared to Girona, currently Atletico Madrid is under pressure from Bilbao who are behind them with a difference of 2 points in fourth place, While Girona, who are ranked third, have a relatively safe distance from Atletico Madrid with 7 points.
Of course, this is a very important match for both teams, especially in fighting for a position in the Champions League next season. However, regarding the statement that both teams have a 50/50 chance of winning the match. Maybe this could be right or could be wrong.
The game of football is very unpredictable, and there is always a chance that anything can happen on the field. However, while considering factors such as team condition, previous meetings, and other contexts, to see a more definite advantage of a team. You noted that Atletico Madrid leads the head-to-head results over Girona, which is not surprising considering Girona's modest pedigree. However, keep in mind that the team's fate and conditions can always change. As you have said, it is very likely that this season Girona will show some changes for the better regarding the quality and performance of their team, and if so, that could make the results of previous direct meetings irrelevant in terms of predicting the outcome of the next match.