First, GC is generally not guaranteed to happen on any object. If you have plenty of heap left,
Java may decide not to waste time doing GC. The only time that GC is guaranteed to happen is if the heap would otherwise be exhausted; you won't get an OutOfMemoryError when there are objects that could be GCd (*)
It is better to talk about objects becoming
eligible for GC, as this happens at a known time, whereas GC itself happens at an unknown later time, if at all.
In your posted code, it is not possible to say whether any of the objects become eligible for GC when the method returns. It depends what the caller does with the returned Collection. If they simply drop the returned Collection, then the Collection will be eligible for GC on return. If they store a reference to the returned Collection, then it will not be eligible, and neither will any of the objects it contains.
Normally, the objects in the Collection would become eligible for GC when the Collection does. But, if there are other references to them, they won't.
Basically, there's nothing special about Collections. All objects are eligible for GC when no-one has a reference to them.
(*) unless you are attempting a single allocation (e.g. big array) that's bigger than all the GCable objects put together.