Another rough comparison pic.
Left to right:
Wargames Atlantic with Wargames Atlantic Hedderheim helmet ~(3rd Century)
Gripping Beast with Wargames Atlantic segmented helmet ~(3rd Century - 5th Century+)
Wargames Atlantic with Gripping Beast ridge helmet ~(4th Century+)
The heads are interchangeable to my eye, just some minor neck surgery required to make them sit in the right place. Until seen side by side the slightly soft detail on the Wargames Atlantic figures was not obvious to me. Heightwise the Gripping Beast is 1-2mm shorter but compensated by the thicker base. (I have mounted them in the photo so that the bottom of the feet are about level.) They are all in a semi- crouch pose so it is not particularly jarring. They are very similar in heft/chunkiness, the Wargames Atlantic sculpts are slimmer and slightly longer in the arms and legs.
The vast difference between the two different sprues is the wealth of parts on the Wargames Atlantic frame and the exceptionally mean paucity of parts on the Gripping Beast version. The extra heads (particularly the pillbox caps and the Intercisa ridge helmets) will be very welcome for modifying the Gripping Beast figures. The heads are less cartoony and of course the armoured bodies allow a more sensibly equipped force for the 4th Century and beyond.