There are some Gigabyte GPUs that are slower than Msi GPUs; there are some Gigabyte GPUs that are faster than Msi GPUs; there are some Gigabyte GPUs that are equal in speed compared to Msi GPUs.
The lifespan is hard to answer, as this is a case-to-case basis. If you take care of your components and don't do anything extreme, mishandling, un-recommended electrical connections, etc., your GPU will last a life time - whether you get a Gigabyte GPU or an Msi GPU.