Take a look at https://github.com/maxomedia/mxm-gulp/tree/support/incremental-less for a newer version of this very problem/approach. Be aware that you can not control the sequence of file imports. In most cases it is in alphabetical order based on file/folder names, but I'm sure this cannot be trusted.
Test repository linked with this question on stackoverflow: https://stackoverflow.com/questions/26628421/incremental-gulp-less-build
Clone the repo, run npm install, then gulp and be amazed, or not, I'm just a readme.