"Solo Adventure in Alan Dean Foster's Humanx Commonwealth"
"A kid's best friend is ... an alien pet?"
Could play as Flinx (the hero of the novel), Maliena or a character of your own. Three possible pets, four possible guardians and several alternate paths, For Love of Mother-Not had fairly good replay value.
Didn't require the GURPS Humanx sourcebook, but it was certainly useful to be familiar with the series. Does require the GURPS Basic Set.