My novel has a lesbian girl, but that only comes in WAY later. How quickly do GL fans expect to see the tag come into the novel? Do they care more about the build-up relationship or the quick wish fulfillment?
That's a really tricky question and has a bunch of factors to keep in mind:
-Is the romance a big part of the plot? (if not, the GL subplot tag might fit better then GL as a genre)
-Is it the MC or a supporting character? (similar to above, or maybe no tag at all)
-Is there buildup for the romance before it becomes "official" (personally, I like the journey towards a good relationship, if done right, just as good as the relationship itself)
If reader counts are important to you for your motivation, adding the tag from the start (and maybe the Late Romance tag so people are informed) might be a good move.
Early exposition and consistent updates seem to be two major factors to get a novel trending and keep exposition high.