I have applied a baseline grid to some text, but there is a gap between the two lines of text. I want the text to sit on the baseline underneath the top baseline. It has a character style applied to it and the point size is 12pt.
I guess your leading is bigger than the step of the baseline, this is why your text can't be aligned to each line of baseline grid. The baseline grid does not override the leading: the text will still try and follow the leading you set, while also obeying the baseline grid.
Just decrease the leading:
Avoid using auto leading for straight text. Always give leading a value. eg 12/14, 12/15. Give text a little breathing room or decrease the point size of the text in order to give the leading a lesser value. Check the character style and make the changes necessary.