What's the best approach if I want to snap to the canvas' horizontal and vertical centre points? Let's say I want to place a guide in the center of both directions for example.
There's no snap to the centre of the canvas in GIMP. However you could modify the grid using Image > Configure grid, then set the horizontal and vertical values to half the image dimensions, then enable snap to grid, and view grid (if you want to see it).
Also, there's a solution for getting guides in the centre. Click on Image > Guides > New guide (by percent). The default is 50% and Horizontal. Repeat for the Vertical guide.