Your translation is almost correct: Depending on the country, I'd replace "Gelsen", because that is a local term. "Mückenstich" is a more generic term in German-speaking countries. And prefix "handeln" with "be-", making it
"Du solltest den Mückenstich behandeln (lassen)."
You could also say
"Du solltest etwas wegen des Mückenstichs tun/unternehmen."
Then, there is another way to say that, although it is somewhat colloquial:
"Du solltest etwas gegen den Mückenstich tun." (colloquial because of the construction "gegen...tun" - one can hardly so something "against" ("gegen") a mosquito bite; one can do something "about" ("wegen") it. But as "gegen...tun" is often used in spoken German and - as such - perfectly fine, I'd like to mention it here.)