Anywhere in the world, having access to transport is desirable; however, try to pick a lot where you cannot see or hear the trains as that may devalue your property, and preferably get a lot on a quiet street. The small, new lots are lovely as they are fresh and low maintenance. However, for the same money you can probably get an old house on a larger lot that in future years would have good sub-division potential and would be more valuable. Also, when you get Phase 1, it means there will be building works going on for a long time still + noise and dust - however, new suburbs look good!
Gosnells seems to be doing very well at the moment and helping the Perth market along. The new developments are obviously on smaller land lots but if you like the area and are happy to live there, then why not go ahead. The Gosnells Central concept is one that is working for the northern suburbs so should continue to add value in the south.