Gwerthu, when present by itself with no other words or phrase, is best translated in English as "to sell" (the infinitive verb form) or "selling" (the present continuous verb form).
The basic, dictionary-form of a verb in Welsh (hoffi, yfed, prynu, etc) is known as a verb-noun (berfenw). As you will see later in the course, verb-nouns can be used very flexibly in Welsh.
The nearest equivalent in English is the 'ing-participle' form of verbs (liking, drinking, buying, etc) but it is often also translated as the 'to-infinitive' (to like, to drink, to buy, etc)." https://www.duolingo.com/skill/cy/Present-2/tips-and-notes