I would suggest consulting with Gateway support to see what your options are. Gateway should be able to provide support, specifications, documentation, and any other details for the Intel® Core™ 2 Duo processor T5450 and the Gateway M-6755. While there may be other processors that are compatible with the Intel® GM965 Express Chipset, if you upgrade to a processor that is not (validated?) supported on that particular laptop, you may be on your own if your system does not work correctly.
Here are few links that may be helpful to you from a processor standpoint: