Do grad schools look at the overall GPA in applications, or do they look at particular courses? I'm wondering whether it'd be worthwhile to spend an extra semester in my undergrad or two getting my grades up. Unfortunately I've taken all the courses relevant to my field, so this would mean using unrelated courses to boost my GPA. 

But I'm not sure if the unrelated courses would actually serve to raise my GPA in the eyes of grad school application boards: would they be pleased with the higher GPA, or would they disconsider it since it was obtained using irrelevant courses?

My GPA isn't too bad, but I luckily can afford to spend another semester or two getting it up if it'd make me a more competitive applicant (and might as well if I get rejected from the schools I want to go to). 

Edit: apologies, I had no idea that some countries don't employ the GPA measure. GPA stands for "grade point average" and is essentially your average grade across all the courses you've taken during your time in undergrad (and possibly Master's).
If this is the US, then it could be either, but the grades that are most important are the ones that relate to your background and aptitude for the program you apply to. Some others may also be weighted fairly heavily. For example, if you apply in CS, then your grades in a writing course might be considered significant, since writing is an important part of CS overall. 

So, what is relevant and what is not depends on a lot of factors and is probably judged a bit differently by everyone who reads your application. But think broadly about relevance. Just because it isn't your major subject doesn't mean it is not valuable. 

But, there is no reason I can see for you to avoid applying now but using additional coursework as a backup. You don't really know how acceptable you are until you get the application in front of some committees. And their view will depend on much more than a few numbers such as your GPA. Letters of recommendation, especially. 

The view outside the US might be a bit different.