What version of llama.cpp are you using (exactly), what command are you using to run the model (exactly), and which version of Gemma are you trying to run (exactly)?

There are open issues on llama.cpp where people have gotten similar <unused49> spam under various circumstances, but the cause is not clear -- e.g. https://github.com/ggml-org/llama.cpp/issues/26088 -- if this can be consistently reproduced, you may want to let them.

Similarly, people mentioned it on some of unsloth's quants a few months back: https://huggingface.co/unsloth/gemma-4-26B-A4B-it-GGUF/discussions/2

As far as general workaround advice: try Vulkan instead of CUDA (or vice versa), try a different version of llama.cpp (especially if you're running either a bleeding edge version or a very old version), and try a different version of the model.