Well, first off, people can say whatever they want. I can say GLP is only good on ap wukong ADC.
Now, to my actual point. It doesn't mean you are a support in the sense the word "support" is used. But the term "support" itself is outdated (unless you want to refer to the position on the map, then it's pretty self explanatory) so I would rather stir away from using it.
A better word to describe mids that use GLP the most efficiently would be "facilitator" in my opinion. Most of the time they are not the main carry in their team, and they serve to facilitate kills in the early and mid game. A veigar mid with glp and Glacial Augment for example is a great champion to facilitate kills whenever your jungler comes to gank. You auto to slow, then you have a much better chance to hit your E, and this way you don't really need your jungler to have hard easy CC, they don't need to be a J4/amumu/(whatever heavy ganker you like) they can also be a more carry oriented jungler and your team won't lack in the CC department.
And when the teamfight phase comes you take the role of the one that zones away from baron/drakes or you make the teamfight a lot more easily maneuvreable for your top and adc that will have more carry oriented champions than you.(if you drafted correctly)
So, I wouldn't say Glacial+GLP makes you a support, but it changes your teamfight role from someone that needs to access the enemy backline and delete people to someone that makes it easier for the rest of your team to play the game. (of course if push comes to shove or if you get a clear cut opportunity you can still go in for the kill because your damage is not that lacking)
Now maybe I am 100% wrong and some high elo person can correct me. But these are my takings on the Glacial+glp approach.