Pokemon is quite a hard game to hack if you are not otherwise familiar with ROM hacking. Fortunately for those wishing to hack pokemon games they usually have lots of very high level tools to change things.
Similarly game maker to assembly is quite a leap (game maker is an odd setup as far as programming things goes).
The other stuff Klarth already took.
Also as the word came up scripting as it applies to pokemon is dealing more with most others would recognise as game events, certainly posts like "I need help with scripting", "scripter wanted" and such like will be for that. To that end if you do go exploring the pokemon hacking world then be aware of this wording change.
Game text scripts then.
Games can be in ASCII and several GBA ones are. ASCII and 8 bit stuff is not that useful for Japanese though (thousands of characters is more than the 256 options afforded by having 8 bits per character*) so some games will use other formats including known ones like unicode, EUC-jp and shiftJIS though this is more common on the DS.
*there are annoying workarounds that some games have used over the years but let us not go there at the moment as they are not common on the GBA and more modern systems.
Custom character mappings then.
You can find these in all sorts of ways, if working on an English or other language that uses the Roman character set you can use a relative searcher. Beyond this there are all sorts of programming, statistical, hackery and other tricks to find it right up to actually just debugging the program. I cover a few in my WIP docs for the GBA and DS http://www.romhacking.net/forum/index.php?topic=14708.0
Once you have the mapping you put it into a table file (for the most part a small text file that lists the custom encodings and what they convert as). At this point you can load it into a hex editor that supports tables or a program that supports tables (aka a script dumper).
There are several of each and though certain non hacking tools will have a tiny measure of support for custom encodings if you are not finding it in http://www.romhacking.net/utilities/
or a site like this one you will likely not get far.
Pointers. Do as they imply and merely point the way to something else. On the GBA they are usually quite simple and as the GBA cart is mainly found at 08000000 through 09FFFFFF (mainly 08 and that allows for 16 megabytes and most GBA games are 16 megabytes or less) you get a lot of 08?????? where the ?????? is the location in the game. In practice the value for a pointer to part of the game as 00123456 will read 5634 1208 courtesy of byte flipping/endianess (ARM processors and indeed most that are not X86 family will do it this way).
That is a vastly simplified version of it though as the GBA cart is also found at several other locations in the GBA memory (seldom used and theoretically you could do a search for 5634 12 and it would also work), things could be transported to main memory (especially if compression is involved) and things could be using far more complex pointers/pointer maths (would be odd but not unheard of).