Perhaps use the line mark with some placeholder data:

http://gosling-lang.org/docs/mark#line

To define a horizontal line, I presume that the data could something as simple as a JSON object that contains two points, one at each extremum of the x-axis (whatever that is for your dataset), with a signal or score value of 2.5:

http://gosling-lang.org/docs/data#json-no-higlass-server

The line mark would connect these two points, giving you a horizontal line.

Details of your approach is missing. I assume, If you are working in R you can try this:

hline = 2.5, vline = 2.5

or the coordinates you are interested in.