Why does ga- not appear in the Gothic past participle?

Why does ga- not appear in the Gothic past participle while ge- does in Old English and German? The study on the Cumulative tendency (CT) which Niwa has made for a long time is found to solve this problem. CT is a universal tendency to strengthen a weakened linguistic unit. So I show in 3. how to solve this problem by CT. The Generative Grammar dealing only with structures could not solve this problem but the minimalist approach has made it possible by introducing semantic features. It so happened that a book by Roberts and Roussou (2003) about the minimalist approach to grammaticalization was published. Following their approach I try to solve this problem in 4. The answer to this problem combines the deficiency of inflectional endings, unstableness of the ga-/ge- prefix and a productive prefix system in Old English and German