Gandul- word origins
Steve, if you don't mind I am going expand a bit here for two reasons. First of all you may have tried to look for the word guandul in the dictionary and could not find it and secondly a thread should provide information beyond what the OP asks so that it becomes a worthy source of info for those reading the threads in this forum.
Depending on your depth of knowledge of Spanish lexicon (vocabulary) specific to Spanish spoken in the Americas many of the words are of indigenous origin i.e.Ta?no, Aztec, Maya etc. and African origin which includes various dialects. Due to the Atlantic slave trade which brought over a million + slaves to the Caribbean and three million + to Brazil needless to say the impact from a language perspective was influential. The vestiges of vocabulary is very evident (IMO if you are truly familiar with words of Latin origin vs. any other word in Spanish) especially when referring to foods specifically vegetables.
Gandul is a word of African origin and it also refers to the seed (la semilla) of the plant. Pigeon peas are also referred to as congo beans or gandule which you will see on the label of cans in the international section of most grocery stores. Gandul is the most common word used for pigeon peas as evidenced by the name of the seed. The Dominican variation "guandul" which is not registered in the dictionary is a classic example of a word variation due to the history of a particular geographical region. In Puerto Rico and other Caribbean Spanish-speaking countries gandul (gandules) is used.
Across the Spanish-speaking world in the Americas there is different vocabulary for the same vegetable due to origin of the people and other historical factors. As you move from region to region you will see this variance. Another example is the word for bean(s) which you mentioned in your original post. Depending on the country the word will vary. Some countries use a more generic word regardless of the type of bean and in other Spanish-speaking countries the word used is specific to the type of bean. For example you will hear: frijol, fr?jol (note the accent on the "i") fr?jol, jud?as, habichuelas and habas.
There may be more but these are the words I am familiar with for beans and I know exactly when to use one over the other. For example in Colombia, Medell?n and the south specifically they say fr?jol (with the accent and stress on the first syllable) as opposed to frijol in the Caribbean. I am sure you heard of the Cuban dish Arroz con frijoles negros.
Be prepared if you go to the Caribbean you will hear of many rice dishes- arroz con guandules (moro) or gandules, arroz con habichuelas, arroz con frijoles negros etc.
-LDG.