I have the "Adobe Helvetica" font installed in my system and I have no problems using the font with the traditional methods, that is, the font is available from applications like "xfontsel" and GTK applications. However, while in some other systems (not mine), I've seen the font can also be selected in GTK+2 applications under the name "Helvetica", I can't do that. The font is not in the list.

Is this normal? Does it happen to you? If you need more info about any specific configuration file in my system just let me know. I'm not sure where to start from.

Did you run fc-cache?

Yes this is normal.

This is one of the fonts that come with X, right? Pat intentionally set up Fontconfig to not show them. There's a comment in the SlackBuild about it, right before the patch that makes it happen:

And from the patch (which modifies /etc/fonts/conf):

If you want to change that, then create a local.conf file with the above two directories, ensure that /etc/fonts/conf.d contains 70-yes-bitmaps.conf instead of 70-no-bitmaps.conf, and then rerun fc-cache. If you then see blocky, non-scaling fonts in unexpected places (such as webpages), then consider changing it back.

No need to run fc-cache. It worked right away. I included those two directories in /etc/fonts/local.conf. I don't know why I missed them in the first place (I had already been poking around /etc/fonts). Thank you very much.