Trees or shrubs, up to 15(--20) m tall, glabrous to variously pubescent, sometimes drying grayish green, brown, gray, chestnut, reddish brown, or with a distinctive orange cast. Branches flattened to tetete or quadrangular, 0.5--15 mm diam.; internodes 0.1--26 cm, smooth or with 2 to 4 longitudinal ribs, sometimes corky (G. alata). Leaves sessile to petiolate, decussate, paired or rarely ternate, isophyllous to a little anisophyllous; blade 0.5--55 x 0.1--20 cm, linear-lanceolate to elliptic, elliptic-oblong, or cuneiform, margins flat or infrequently crisped; secondary veins visible or sometimes not visible below, eucamptodromous, spreading and arched or infrequently nearly straight and ascending at an acute angle; domatia absent or present as pilosulous or hirtellous, tuft- or crypt-domatia in axils of secondary veins; petioles 0.2--10 cm , smooth to occasionally furrowed or rarely winged, smooth or usually encircled on sides and below by ridges or wings, these often extending onto stipules. Stipules 0.1--75 mm, tubular and open at top in bud to fused at top and calyptrate, cylindrical to infrequently funnel-shaped, entire or splitting along 1 side to form a spathaceous structure, splitting on 2 sides to form 2 usually intrapetiolar segments, or rarely splitting on 4 sides into 4 spathulate parts (G. furcellata, G. microphylla), drying membranaceous to coriaceous, variously caducous, deciduous through fragmentation, or persistent, usually with 4 longitudinal ribs or wings arising below petioles and extending along sheath to lobes, sometimes with 2 additional ribs or wings; apex entire or with the 1--4 incisions described above, marcesent (i.e., becoming hardened) to fragmenting or infrequently persistent; lobes none or 4, up to 20 mm, deltate to filiform; additional setae none or several to numerous, to 20 mm. Inflorescences terminal on developed stems, sometimes borne on short axillary stems, or rarely axillary or terminal on supraaxillary stems, cymose to compound-cymose, paniculiform, subcapitate, reduced to 1 or a few flowers, or rarely spiciform, erect to occasionally pendulous, sessile to pedunculate, green, white or pink, bracteate or rarely ebracteate (G. alata); peduncle to 15 cm; branched portion up to 25 x 30 cm, pyramidal to corymbiform-rounded, subglobose, or rarely cylindrical, branched to as many as 6 orders, lax to congested; axes dichasial or rarely scorpioid (G. divaricata); bracts subtending basalmost axes triangular to linear or sometimes resembling reduced leaves or stipules (G. microphylla), rarely grouped into an involucre (G. cuneifolia, G. microphylla); bracts subtending higher-order axes up to 30 mm, deltate to linear, ovate, or trifid, usually grading in size and shape between basalmost bracts and bracteoles; bracteoles reduced to developed, inserted on pedicel and/or at base of calyx, rarely enlarged and bright white (G. phyllosepala, G. phyllostachya). Supra-axillary reproductive branches rarely present (G. diversifolia, G. inflexa), paired, flexuous, with leaves well developed to reduced. Flowers sessile to pedicellate, 4- to 5(6)-merous, dioecious or bisexual and heterodistylous. Calyx limb cup-shaped, urceolate, or campanulate, 1--20 mm wide, outside glabrous or variously pubescent, inside sometimes with a distinct ring of trichomes, truncate to denticulate or lobed, lobes rounded to triangular or linear, equal to markedly unequal, up to 30 mm, sometimes petaloid in color and texture (G. phyllosepala, G. phyllostachya); corolla white or infrequently pink, red, orange, or blue, clavate to rhomboidal or obclavate in bud, at anthesis salverform, infundibuliform or campanulate, 2--30 mm, outside glabrous or variously pubescent, inside glabrous or villous inside tube, tube entire or rarely fenestrate (G. cooperi), lobes 1--10 mm, narrowly triangular to ovate-oblong, glabrous adaxially, apically rounded to acute or rarely expanded and cucullate (G. cooperi), abaxially smooth or rarely with a thickened subapical appendage (G. edentata); stamens inserted in corolla tube, anthers 1.5--4 mm, narrowly oblong, dithecal, dehiscent by longitudinal slits, sessile or with developed filaments to 8 mm, in long-styled flowers positioned below the stigmas and included to partially exserted, in short-styled flowers position above the stigmas and included to exserted, in staminate flowers positioned near middle of corolla tube to exserted, in pistillate flowers staminodes present or absent, positioned in lower part of corolla to corolla throat. Ovary superior, 2-locular, 2-celled, with 1 erect basal ovule in each cell; style filiform, glabrous or pubescent, stigmas 2, linear-clavate, often flattened, in long-styled flowers these positioned above the anthers and exserted, in short-styled flowers these positioned below the anthers and included, in staminate flowers pistillodes developed or reduced and positioned in lower part to middle of corolla tube, in pistillate flowers these developed and positioned in lower to upper part of corolla tube. Fruit a drupe, blue to violet-black or reportedly sometimes whitened, globose to ellipsoid, obovoid, or didymous, smooth or infrequently ridged, 5--28 x 5--16 mm, glabrous or rarely pubescent; pyrenes 2 per drupe or sometimes 1 apparently by abortion or incomplete pollination, spherical or hemispherical to wedge-shaped, ± smooth to finely fissured, rugose, and/or deeply fissured, endosperm entire to invaginated or ruminated.