GBA ROMs are just one big contiguous chunk of data. They don't have file systems like ISOs and DS ROMs. You'll have to hunt through 'em for the data you're looking for.

It depends on what you're looking for, really. Obviously you'd use Tile Molester for graphics and a hex editor (I recommend WindHex) to hunt down text. It helps if you use an emulator with debugging functions, like VBA-SDL-H or (preferably) NO$GBA: you can then find the data you're looking for in memory as the game is running, set a write breakpoint to the location where the data is written, then examine the trace to find out where in the ROM the data was loaded from.