Define self-sustaining. If it's about membership, then yes they can, if they have decent results over a long period of time (at least 20 years). Years on end of being garbage is not going to win them a lot of support though, which is why the AFL had to intervene with the Suns and give them a rescue package. Despite this, both the Gold Coast and Western Sydney are booming in terms of juniors playing Australian rules, so given another generation and no long periods of ineptitude, they will get plenty of members.The bigger question is whether Gold Coast and GWS can be self sustaining.
If it's about attendances at games, both of them have a serious issue that's out of the AFLs hands: their catchment areas have poor transport connectivity in general, and especially to their home stadia. This will be improved for the Giants in a decade when a metro line is built directly connecting Homebush to Parramatta, at which time Parramatta will be a light rail hub also. The Suns are not as fortunate, since a light rail extension to Carrara seems to be well down the list of government priorities, and the long-promised ferry service still hasn't manifested. So I can see the Giants selling out every game by the early 2030s, provided they don't suck, whereas the Suns will find it tougher.
No one seriously advocates getting rid of the Swans or the Lions, because they've been around for decades now, have both had periods of success recently and have decent connectivity to their stadia. Although there were whispers about their viability when they had sustained periods of ineptitude (early 90s in Sydney's case, mid 2010s in Brisbane's). An even better example is the Melbourne Storm, who have some of the highest crowds in the NRL despite being in AFL territory, because they've had almost constant success for 25 years and an easily accessible stadium. If they can do it, the Suns and Giants can too.