Bags and caps: Depends. If you can flatten the bag, to where it fits on the platen, and it's made of a material that absorbs ink, yes. Caps can be done with a special Cap Platen available from Stitch City.
Gold and silver: The printer uses CMYK process color printing, it can print a gold and a silver, but it is the same gold and silver you would get if you printed to your desktop printer. If you are looking for metallic inks, the Brother doesn't have any, nor do any of the DTGs to my knowledge. Most metallics are done using screen printing or foil printing.
Babies shirts: Yes, I do little baby bibs, shirts, socks, caps, etc, usually I lay them flat on the platen and print, I have a printed grid set up on my main platen so that I can get precision printing/placement.
Sleeves: Yes, you can either use the sleeve platen, or you can play with a regular platen so that the print is positioned on the sleeve. I can't justify the cost of a sleeve platen, so if someone is really annoying me enough to put it on the sleeve, I will print it on my regular platen trying to get it flat and tape it down, then align my graphic using my printed grid system.
Oversized prints: Stitch city has a dual print platen for doing big images, I haven't seen it in use except for on their video. I try not to exceed the max size of my 15"x15" press. Some of the epson based
dtg's have custom platens for printing bigger, however I think most all overs are done via screen printing.
Judging by your questions, screen printing is probably the most versatile solution for your needs, however it is not made for small quantities, and it is a trade, there is a learning curve, and you will have to spend a lot of time learning it.
To GT-541 owners :) 
