Yes, the gas strut (or gas spring as you call them) are essential to pushing the tailgate up and holding it there. Without them the tailgate is very heavy. With age sometime the gas strut leaks and lose some of their pushing power. On manually operated lift gate this will result in the liftgate falling back down (not staying in the up position). On powered tailgate, this will sometime result in the motor thinking that the tailgate hit something and will cause the tailgate to stop and go back down, or sometime "fall" back down.
I think you mentioned your gas strut were dirty in a previous post, oily looking leaks onto which dirt collects is sometime a sign that a gas strut is leaking and losing its charge. Replacing gas strut is a fairly inexpensive job and would be a good place to start troubleshooting your issue.
