If my mom Grandmother was Spaniard, does that make my mom 25 percent Spaniard? What percent does that make me?
If your Great-grandmother was 100% Spanish. Then your mother is 25% Spanish providing her grand-father (Your maternal great-grandfather) and father were 0% Spanish. If this is the case then, genetically speaking you are around 12.5% Spanish, providing that your father is also 0% Spanish.
However you don't inherit exactly 12.5% from each great-grandparent. You could have inherited a lot more 'Spanish Genetics' from your great-grandmother than average or just as easily much less.
From a genealogical standpoint you are 12.5% or 1/8 Spanish. Again, providing that's the only grandparent with any Spanish lineage.
Now, if you want to go further in to the genetic side then you need to know how DNA inheritance works.
It is not that you "get half your genes" from each parent, and thus a quarter from each grandparent, it is that you inherit the versions of the genes (we all have the 'same' genes).
The method that determines which traits are inherited from each parent by the offspring is known as homologous recombination, and this process is (essentially) random, and thus you end with ~50% of your traits (the alleles of the genes) from each parent, and ~25% from each grandparent, so you are right in this respect.
This is, however, a generalization; due to the chance nature of the recombination it is entirely plausible that you may inherit more traits from one grandparent in comparison to another, but this is unique to each individual (with the exception of course of genetically identical twins).
The above diagram gives a good representation of how genes are inherited. However there are a lot more genes than 'blocks' shown.
There are an estimated 20,000-25,000 human protein-coding genes. https://en.wikipedia.org/wiki/Human_genome