Gallery being empty may probably be something to do with gst-video-thumbnailerd or trackerd messed up. There could be broken packages and/or simply trackerd is not setup properly.

This will show you a list of installed programs (both hidden and visible). If you look at the very top banner of the output you will see that when any of the first two characters are capitalised, it means there was an issue (hence broken):
Also tracker-stats (another terminal program) shows any files that were found via tracker and has been added to the database. Standard maemo programs such as gallery, mediaplayer, etc depends on tracker database.

Applications not working could be broken depending on what is broken. It is difficult to know where is broken without attempting to run the program through terminal for instance to see where it would spew out the error.

Battery not calibrated could mean many things. It doesn't help if you don't tell us what sort of setup does your N900 have. The same case goes likewise as above for other programs. Nobody knows your exact setup (both hardware and software) except for you. It would help if you would tell us what you have. What programs are broken, what sort of battery do you have or did you experiment with some software that may have broke the calibration or something.

The bulk of these things does require lots of patience and background knowledge. Otherwise, it can be a long and tedious process in getting the various issues you have sorted out.