Try this hack :-) Gulp v3.x Hack for Async bug
I tried all of the "official" ways in the Readme, they didn't work for me but this did. You can also upgrade to gulp 4.x but I highly recommend you don't, it breaks so much stuff. You could use a real js promise, but hey, this is quick, dirty, simple :-) Essentially you use:
var wait = 0; // flag to signal thread that task is done
if(wait == 0) setTimeout(... // sleep and let nodejs schedule other threads
Check out the post!