【Gather】 ,  【collect】 ,  【assemble】 ,  【congregate】  mean to come or to bring together so as to form a group, a mass, or a unit.

The same distinctions in applications and in implications characterize their derivative nouns gathering, collection, assemblage (which see) or assembly, congregation.  【Gather】 is the most widely applicable of these words; it may be used in reference not only to persons and objects but to intangible things.

In certain phrases 【gather】  acquires additional specific connotations; thus, gathering flowers or crops implies plucking and culling as well as bringing together; gathering a ruffle implies a drawing together or into folds on a thread; gathering one’s wits connotes an effort at concentration or at mustering or rallying mental forces.

【Collect】  is often used in place of  【gather】 with no intended difference in meaning. But  【collect】  may convey, as  【gather】  does not, the ideas of careful selection or a principle of selection, of orderly arrangement, or of a definitely understood though not always expressed end in view; thus, to  【collect】  butterflies implies a selection of specimens and, usually, their cataloguing; to  【collect】  books (as in book collector ) implies a choice of books with regard to some such principle as rarity, beauty of binding, or authorship.

There is a subtle difference between to  【gather】  one’s thoughts, which often merely implies previous scattering, and to  【collect】  one’s thoughts, which implies their organization; there is also a difference between to  【gather】  money, which may mean merely to accumulate it, and to  【collect】  money, which usually suggests either raising a fund by gifts, subscriptions, and contributions or taking action to obtain possession of money due.  【Collect】  and collection are often preferred to  【gather】  and gathering when various things are brought together; thus, a jumble or an omnium-gatherum is a miscellaneous collection rather than a gathering;  【collect】  rather than  【gather】  enough chairs for all the guests to sit down.

【Assemble】  stresses more emphatically than either  【gather】  or  【collect】 a close union of individuals and a conscious or a definite end in their coming or in their being brought together. It is used chiefly in reference to persons who 【gather】 together, either of their own will or at the call of another, so as to form a group or body that will unite in action or join in counsel or discussion.

In reference to things  【assemble】  implies an agent who collects them in order to unite them into a single body or structure or into a distinct and isolated group; thus, the assembly department of an automobile plant is the department in which the workmen build the cars by assembling the component parts made in other departments or in other factories.

【Congregate】 implies a flocking together into a crowd, a huddle, or a mass.

Congregation is specifically applied to an assembly meeting for religious worship, but it usually retains the suggestion of a crowd that has flocked together.