If you hosting your application, then it would not be problem. As you said other application like gmail etc uses the same port 80, i.e. means gmail server keeps listening on the port 80. When we access the gmail, any port is selected (1024 <) on client side. these ports are usually called as ephemeral ports
So when you access gmail, port say 41667 on your machine opened and connected to port 80 of gmail. port 80 inturn pass to other available port and keeps listening so many user at the same time can access gmail.