I find CTF better for getting more GP and XP than standard elimination, especially if you get a couple of flag caps of flag recoverys.
One method for ranking up fast that i thought of yesterday but haven't tried yet is this though, you might wanna give it a try:
Have you ever joined a server only a couple of minutes from finishing and then at the end, with only a couple of kills and 2 minutes played, you get aroun 70-80 XP and similar with GP. Well, this could be used for fast ranking up.
If you grab a piece of paper and a pen, start joining a few different servers and note down the number of the server and the time left to play (have a clock or stop watch to hand also). Once you have a few servers jotted down the say finish in 2 minutes, 5 minutes, 8 minutes and 11 minutes, you can then start with joining each server just in time for the round end and some free and easy XP points. Once you have got the finishing screen and points for the game 2 minutes, you can move into the room that was finishing in 5 minutes (that now will be just 2-3 minutes from ending)...and so on...and so on.
As i said, i haven't tried this but have experienced it a couple of times where i've joined a server, just for it to end shortly after but still get a substantial wad of points, even with low or even NO kills.
Try it out if you want, and maybe report back with your sucesses and failures.
As i said at the start though, just play the mode you find the most fun as you will at least get more skilled at the game and learn the maps and weapons better. From my experience (limited) Elimination offers the pooest rewards for taking part in, although it is one of the most popular clearly.
Hope this helps and isn't just a waste of time
Rad.