You couldn't customize directly the gridlines like you want. Options to influence the gridlines could be the type of gridlines and the scale of the axis especially the interval-steps.
Another approach would be to create another expression for the grid whereby the bars are hidden or transparent - argb(0,0,0,0) - or you used a second bar-chart which will be overlapped from the first one and you adjust there various options, like the expression, scale and other layout-settings.
Further if your chart is quite static you could consider to replace the grid with a background-image or presentation-lines from tab presentation.
