Complex sentence: As a Germanic tribesman, Ricimer could not assume the Imperial throne himself, but as "magister militum" he gained infuence over the Germanic peoples occupying Gaul, Hispania, and Northern Africa.
Split into simpler sentences: As a Germanic tribesman, he could not assume the Imperial throne himself. On the other hand, his position as Supreme Commander gave him prestige and influence over the other Germanic peoples occupying Gaul, Hispania, and Northern Africa.