Well, from one Gentoo user to another, here's a bunch of things you can try or check (although the one time I had a problem like this, it affected all OpenGL programs, not just Löve):
Just in case, make sure you've done the obvious stuff with eselect ("eselect opengl set nvidia"). Make sure "nvidia" is included in the list returned by lsmod (that is, that it's the driver being loaded by the kernel).
Make sure that the nVidia driver is actually the one being loaded by OpenGL ("glxinfo | grep version" should give something like "OpenGL version string: 3.3.0 NVIDIA 304.135"—my card is older, but it's the "NVIDIA" that's important). If you get something else, you may have a bad LD_LIBRARY_PATH or a missing symlink.
Re-emerge opengl, libsdl, and nvidia-drivers. Just in case.
Try one or more of the love-0.10.2 ebuilds from various overlays (there are at least three, per
http://gpo.zugaina.org/games-engines/love ) to make sure you're not getting problems due to lack of ebuild maintenance in the main tree or missing requirements information.