It's not quite as simple as it is in Italian, but there are some general guidelines. In nouns denoting people or animals, gender usually corresponds to sex (e.g. gŵr, tarw (m) ~ merch, buwch (f)), but there are also some nouns which have a fixed gender irrespective of the sex of their referent (e.g. plentyn (m), cath (f)). Some word endings are typically masculine (-yn, -wr; -ad, -deb, -der, -edd, -had, -yd, etc.) or feminine (-en, -es, -wraig; -aeth, -as, -fa, -ell, etc.). Names of days, months, seasons, compass points are masculine. Names of countries, regions, towns, rivers and trees are usually feminine. Verb-nouns are masculine. Most collective nouns are feminine. Apart from these general rules, it's best to learn the gender of each new noun you learn (learning it together with the article or, better, an adjective may help -- I'm sure you know that feminine nouns take soft mutation after the article and cause soft mutation on a following adjective). Also bear in mind that masculine nouns are much more numerous than feminine ones, if you have to make a guess.