I really recommend you read and listen to stories like this to keep you motivated rather than getting ink. If you get this "がんばる" tattoo, it will be the number one deterrent in ever dating a girl here. They will be too busy snickering to take you seriously. You're going to have to be very, very handsome to get away with something as silly as that permanently etched on your body.
I don't think the choice of word is near as bad as the choice of having a plain katakana and/or kanji tattoo. There are things we foreigners can do differently than Japanese males and still get a girl, but that is simply not one of them. I once had an all white pull over sweater with a red circle on the front and 日本 written on it. It was a nice sweater. It was very simple and far less loud than your average shirt with American flags on it and such. I thought that would be passable even here, and I had some awareness about issues with the flag etc. Nope. Nobody liked it. They laughed at me.
A really funny story was that of a girl who lived next door to my grandmother. She got 愛 or something tattooed on her stomach. Then she started to get doubts about if it really meant love. She was asking me, when her sister blurted out they all thought it means love. I said yes, indeed, that was love. But she would not believe me, since her sister let the cat out of the bag before I confirmed it. So she lived on in doubt. Its worse than a bad haircut, and how does that help you がんばれ？
My advice: Get a picture tattoo that is both cool and reminds you to study. I have no idea what that would be though.
I hear ganbaru quite a bit.
Me too. But I lean toward def's opinion. My thinking is that its a little too vague and in vague way more than any English translation would be. Probably the best translation I can think of is "to persevere", but even that does not quite capture how vague it is. Japanese and English both depend on context, but the way context is demanded there is special.
Edit: I see I missed a page. Oh well. The all kanji is definitely better, but I still don't recommend it for anyone who has any interest in hooking up with a Japanese woman. A picture or original piece of foreign culture would be better (even having a MOM tattoo). No ink is best though.
Another word of advice. If and when you come to Japan, you will have trouble going into pools and gyms because of the ink. I am not kidding you. You may well be asked to leave if anyone sees it.
Considering that, a ring or necklace could probably be ordered made with the kanji you want on it. Same effect and it comes right off. Probably the same price too.