Game
designers have gained experience in 3D modeling over numerous years, are now developing characters who are difficult to distinguish from real people.
A. designers have gained experience in 3D modeling over numerous years, are now developing characters who
I do not see any grammatical issue with this option; However, meaning wise there is some miss- the two actions are not quite parallel IMO
B. designers, gaining experience in 3D modeling since numerous years, are now developing characters, which
The present participle does not give the right meaning as if designers were gaining experience at the time they were developing characters
C. designers have gained experience in 3D modeling over numerous years and now developing characters that
Missing parallelism -there is change in verb i.e "have gained experience" and "are now developing" .. so it is better to use the main verb
D. designers gained experience in 3D modeling since numerous years and are now developing characters who
Verb issue - simple past is inappropriate IMO; With since we need " Have gained"
E. designers, having gained experience in 3D modeling over numerous years, are now developing characters that
Best in the lot- "having gained experience in 3D modeling over numerous year" modifies designers and connects well with meaning with the main verb i.e after gaining experience --are now developing ..
(E) for me