There's not a certain group or individual who decides (as there is in, for example, France). Often the "decision" is based on existing words--nouns that end in "-chen" are neuter, so any new "-chen" word will be neuter. Most native speakers have a feeling that nouns that end with -e should be feminine (with certain glaring exceptions like "Junge" [boy]),
so new nouns ending in "-e" usually end up feminine. There's supposedly an often-broken "rule" that foreign words that make their way into German should be neuter, but most Germans I know say die Cola, although I believe it can also be das Cola. But why the heck is it die Couch??? As for "-er" words, the preponderance of words ending in "-er" in German is masculine, especially for those where "-er" has the same meaning as in English: "that/who which performs an certain action," so it's natural for native speakers to treat words like "Computer" as masculine
Many years ago, I took a survey for someone's PhD dissertation which investigated whether there is a "native" understanding of gender in English. I recall that most people thought salt was feminine and pepper masculine--they are neuter and masculine in German--and that knife, fork, and spoon were masculine, masculine, and feminine--they are neuter, feminine, and masculine in German.
Interestingly enough, while sun is feminine and moon is masculine in German, the genders are reversed in French.
A good question--I wish I had a good (concise) answer!