Q:

Open-source tag vs gratis

This question is just the latest of many where I have had to say something along the lines of

By the way, you specified open-source, but unless you plan to
  change the code, then gratis will open more possibilities if you
  are just looking for something that you don't have to pay for

I can only think of two reasons for requesting open-source:  

you are going to fork the code
you want something where the code has been peer-reviewed, or can be; for instance, for security apps.

There are more gratis than open-source apps, so ought we to consider a pop-up when someone uses the open-source tag? 

A:

We already have something similar. If you want some software free of financial costs, wouldn't you usually start typing "free" in the tags input? Try it and see what happens: both your tags appear, together with their excerpts. Rather a pop out than a pop up, but I'd say by that it's clear what each of them means.

Don't you think this is sufficient? If people don't read, that can't be helped.

