If you've already got guards installed, standard gutter clips often can't grip the way they're designed to, since there's a mesh or hood surface in the way instead of a bare gutter edge. Some manufacturers explicitly warn that hanging lights with standard clips over a guard system can affect the guard's warranty, so it's worth checking before you improvise a workaround.
The better options are shingle-mount clips that attach to the roofline above the guard instead of the gutter itself, or guard-specific light clips designed to work with the mesh surface. Either avoids disturbing a system that, as covered in , is already doing a specific job you don't want to compromise for a few weeks of lights.