We have all the signs, but when a particular sign does not have a planetary representative, then that energy has one less conduit to manifest itself in reality. For me, that's Gemini.
I have no Gemini planets, but my natal Mercury has many wonderful aspects to other planets (thank God for that!). My best friend since primary school has a Gemini stellium and most of my chums are air dominant (I have a lack in air). The Universe has its wonderful way of making sure that we get what we need, i.e. a chart lacking in fire and the native is likely to find him/herself surrounded by dynamic individuals.
For me, meeting all these Gemini pals is helping me to become more aware of my natural tendency to be on the quiet side. They remind me to be more communicative, that no man is an island; we are social beings who need to connect with others. I also tend to take life way too seriously (my Sun-Saturn influence and a heavily-tenanted 8th house), therefore encountering these individuals allow me to see the joy of living in the moment and to lighten up.
For those who are on the other end of the spectrum, i.e. prone to gossiping and meaningless chattering, seeing another person suffering from verbal diahorrea could reflect their garrulous side.
So really, it's a matter of knowing whether you need to display more Gemini traits or to suppress the excess. Which one are you?