It extends for 118,144 hectares (it is one of the largest Italian protected areas). The four Tremiti islands (marine reserve) are part of the park. Inside the park there is the Umbra Forest. The Gargano is mainly made up of sedimentary rocks, limestones and dolomites, dating back to the Cretaceous and Jurassic periods, mostly stratified and affected by the phenomenon of karst dissolution. An exception is Punta Pietre Nere, a mass of dark volcanic rocks dating back to the Triassic, emerging on the beach of Lesina. The karst phenomenon, produced by the action of water and carbon dioxide on the limestone rocks, has "sculpted" the landscape in various ways. Along the entire margin of the limestone block there are large erosive furrows which, with a radial trend, head towards the sea or the Capitanata. These are rocky ravines (linked to the phenomenon of "dry valleys" or "valleys"), caused by mechanical and karst erosion.

Various habitats are found in the Gargano national park: beech forests in the interior and on the northern side, Aleppo pine forests along the coasts, large extensions of Mediterranean scrub, not to mention the oak groves where turkey oaks and holm oaks abound, mixed woods rich in ashes, ash trees, elms, hollies, chestnut trees, maples, oaks, beeches, etc. The undergrowth is populated by numerous essences: ferns, brambles, dog roses, cyclamens, edible and poisonous mushrooms, etc. On the slopes exposed to the sun, pears, melastres and hawthorns grow surrounded by mastic bushes, juniper, thyme, brambles, prickly pears and the particular "devil's tree" (carob). In the foothill area the vegetation changes radically and the steppe predominates, rich in prickly pears, asphodels, ferules, euphorbias, irises; in which a very particular mushroom, Pleurotus eryngii, grows. Everything is interrupted here and there by olive groves, almond groves, vineyards and wheat fields. Other particular environments of the Gargano are: the marshes of Federico II, marshy areas of Frattarolo and the Salso Lake Oasis, divided into two areas: one where the marsh reed, the cattail, the eucalyptus, the water lily reign supreme and the other where the flora predominates xerophilous or glasswort, rushes, tamarisks, etc.,the coastal lagoons, characterized by an infralittoral forest that grows on the thin strip of sand that divides them from the sea (called "island") and in which the Cistus di clusio and the numerous reed thickets that surround the banks grow. In the innermost areas of the promontory (forests of Ischitella, Manatecco, Ginestra, Sfilzi, Umbra, Bosco Quarto, Umereta delle Ripe) there are large forests of beeches, holm oaks, turkey oaks and, sometimes associated with oaks, elms and ash trees. Other woods of naturalistic interest are also those of Monte Sant'Angelo, Monte Sacro (Mattinata), Spina Pulci (between San Nicandro Garganico and Cagnano Varano. On the coast, however, the Aleppo pine forests dominate, approximately 7,000 hectares which alternate with the Mediterranean scrub, rich in mastic formations, phillyrea, multiflora heather and strawberry tree.