Platform games are generally tile-based. So use a 2D array to store your map tiles. You only draw the tiles that are visible within the viewport (what the play can see on the screen). You'd basically have a map pointer that keeps track of what position in the map data it should start drawing. Using different layers (different Z-depth) you can create parallax scrolling backgrounds.

Don't use JPanel for drawing all these graphics. I never heard of GCanvas (i haven't done games in java) but if its designed for drawing graphics and game sprites, then it'll probably already be setup for double buffering.