Yes, you don't need to/want to duplicate existing values... it's just more/uncessary/duplicate CSS for the browser to load.
Just this:
.g-grid {
align-items: center;
}
would set ALL g-grid's to have their blocks set to "align-center" which you probably don't want. So instead create an additional selector that you can add to your Section's classes in Gantry like:
.g-grid.custom-center {
align-items: center;
}
and then you can add the Class "custom-center" (or whatever you want to name it) to the Section's Settings > Classes field to then center all the Blocks/Particles therein