I speak Russian, which is gendered (masculine/feminine/neuter) as my native language.
> what utility do you think a language gains from gendered nouns?
None whatsoever. When speaking of things that don't have any meaningful gender, it makes things unnecessarily complicated. When speaking of things that do have physical gender, it often forces you to make assumptions about it even when it's completely irrelevant.
> ow do you know what gender a noun is? I assume there are "themes" that might inform you. But if you're unclear, how do you do find out?
In Russian, it mostly follows the spelling of the words - i.e. if a word ends with a certain vowel, it's feminine, except if ... etc.
Sometimes it doesn't work that way, mostly with loanwords. Some loanwords so obviously don't match any "genderizing" pattern, that they basically end up with neuter by default.
In other cases the historical form of the word did match a pattern, and was assigned a gender accordingly - and then changed (e.g. by re-loaning it in a more accurate spelling), and no longer fits. When that happens, people will use the more "appropriate" rather than the "right" gender in colloquial speech, and eventually it becomes the new standard, collecting the mismatch.
A good example of this is the Russian word "coffee". When it was first loaned back in 18th century, it was "kofiy" - and in Russian, that is definitely masculine. Eventually it got re-loaned as "kofe", which would normally be neuter; but the masculine gender assignment stayed from past spelling. In the dictionaries, that is - in practice treating the word as neuter became one of the common incorrect colloquialisms, just because it doesn't "look" masculine. Language purists fought this for several decades, and eventually lost: it's still nominally masculine, but neuter is considered an "accepted variant" in modern dictionaries.
> how are new genders decided? I'm struggling to think of an example as "laptop" presumably has the same gender as "computer", but it must come up sometime
For new words, by their spelling, as described above. So "laptop" is masculine because it ends with a consonant, for example.
Personal names are an exception to this stuff. As in, when a foreign name is used in a context where gender cannot be guessed, people will often apply the usual rules (and then often get it wrong). But once context is established, it's properly accounted for.
On the other hand, for place names, gender is automatically assigned by the usual rules. So to a Russian speaker, New York and Texas are masculine, while California and Florida are feminine.