Breton's answer is not wrong. I like his explaination. However that translation is just a literal translation. The sentence "She acts lonely" is particularly strange.
The speaker has observed Susan, and interpreted her behavior/action as a sign of feeling lonely. It's different from らしい、よう、そう where there are uncertainties. With がる, you guess with certainty. That's why it's quite uncommon in normal conversation, and more suited to be used in narrations. In animations and such, it's more common to translate them as "she feels lonely, she seems lonely, she is lonely" but with the implications that the speaker observed her and noticed how she acted the loneliness. To be honest, I'd prefer "act" as a translation, but this verb with some words, sounds strange/uncommon.