X label in GraphicsColumn is cut off

PlotRangePadding option can give you more space around your graphics objects.

GraphicsColumn[{Plot[Sin[x], {x, 0, 10}, PlotRange -> All, 
   AspectRatio -> 0.3, Frame -> True, Axes -> False, 
   ImageMargins -> 20], 
  Plot[Cos[x], {x, 0, 100}, PlotRange -> All, AspectRatio -> 0.3, 
   Frame -> True, Axes -> False, ImageMargins -> 20]}, Epilog -> 
  Style[{Text["Frequency", Scaled@{.5, .02}], 
    Rotate[Text["Power", Scaled@{.03, .5}], 90 Degree]}, 14, 
   FontFamily -> "Helvetica"], PlotRangePadding -> {0, 70}, 
 ImageSize -> 500]

An alternative using Labeled:

Labeled[
 GraphicsColumn[{
   Plot[Sin[x], {x, 0, 10}, PlotRange -> All, AspectRatio -> 0.3,
    Frame -> True, Axes -> False, ImageMargins -> 0], 
   Plot[Cos[x], {x, 0, 100}, PlotRange -> All, AspectRatio -> 0.3, 
    Frame -> True, Axes -> False, ImageMargins -> 0]}, 
   Alignment -> Left], 
 {"Frequency", "Power"}, {Bottom, Left}, 
 RotateLabel -> True, 
 LabelStyle -> Directive[Bold, FontFamily -> "Helvetica"], 
 FrameMargins -> 0, ImageMargins -> 10]