[RadioKid]

Does "GA" introduce something new to listener ?

Yes, sometimes GA introduce new thing like "OJIISAN TO OBAASAN GA IMASHITA". But I think it is not always true.

In "Door GA SHIMARIMASU", the door is not new for the listeners.

In Microsoft WIndows, sometime it says "XXXX WA loginSHITE-IMASU." which is not natural for me. I think it should be "XXXX GA login SHITE-IMASU.".

"GA" is difficult subject to discuss on. There are many types of "GA" used in Japanese like "SORE GA WATASHI NO IITAI KOTO DESU:it is what I want to say.", "WATASHI WA ANATA GA SUKI-DESU; I like you." etc.

In the japanese dictionary, "GA" is explained very in detail.