The only free and comprehensive online etymological dictionary of the Spanish language
gracias (Interjection) "thank you"

From the Latin statement gratias agere "to give thanks," but more literally "to do thanks." Gratias was the accusative plural of gratia (see gracia). For the etymology of agere, see -igar.
gracioso (Adjective) "funny;" "pleasing"

13th cent. From Latin gratiosus "popular," "pleasing" from gratia "favor" (see gracia).
grado (1) m. & f. (Noun) "step"

12th cent. From Latin gradus 'id.' From Proto-Italic *grad- 'id.' From Proto-Indo-European *ghred(h)- 'id.'

In the 13th cent. also with the meaning "dignity," an analogy of a noble's gracious stride, which is the origin of the surnames Grado, Grados, Grade, Grada, Gradera, Gradín, and Gradillas; as well as the name El Grado for towns in Oviedo, Huesca, and Segovia.

Romance: Western Vulgar Latin: Portuguese grado, Catalan grau, French grade, Italian grado; Eastern Vulgar Latin: Romanian grad
grado (2) m. (Noun) "wish," "preference"

12th cent. From Latin gratus "pleasing." From Proto-Italic *gwrāto- 'id.' From Proto-Indo-European *gwr̥H-to- "praised (in a song)." From *gwerH- "to praise."

Romance: Western Vulgar Latin: Portuguese grado, Catalan grat, French gré, Italian grado

Italic: Oscan brateis "mercy," Vestinian brat 'id.,' Paelignian brat 'id.'

Indo-European: Indo-Iranian: Sanskrit gūrtí- "song of praise"
grado (3) m. (Noun) "grade"

12th cent. From Latin gradus "step," in the sense of steps forming degrees of quality or proximity. For a further etymology see grado (1).

Romance: Western Vulgar Latin: Portuguese grado, Catalan grau, French grade, Italian grado; Eastern Vulgar Latin: Romanian grad
grafia f. (Noun) "orthographic symbol"

20th cent. From Greek γρᾰ́φειν (gráphein) "to write." From Proto-Indo-European *gerbh- "to scratch."

Indo-European: Germanic: Old High German kerben "to carve," Old English ceorfan 'id.' (English to carve); Balto-Slavic: Old Church Slavonic žrěbii "allotment," Russian žérebej "cut off piece"
grande (1) (Adjective) "great," "large"

11th cent. From Latin grandis 'id.' Of unknown origin.

Romance: Western Vulgar Latin: Asturian grande, Portuguese grande, Galician grande, Catalan gran, French grand, Italian grande; Sardinian: grande
grande (2) (Noun) "nobility," "magnate"

A metaphorical extension of grande (1).

Also the origin of the surnames Grande and Grandes.

Grande de España "highest noble." Said of those who enjoyed the greatest privileges of the realm before the king and queen.
grandioso (Adjective) "grandiose"

Very early 17th cent. From grande and -oso, an adjective-forming suffix.
gratis (Adjective, Adverb) "free," "freely"

Early 17th cent. Borrowed from Latin gratis 'id.,' earlier gratiis "by grace," "by friendship;" ablative of gratia "grace" (see gracia).

Romance: Western Vulgar Latin: Portuguese grátis, Catalan gratis, French gratis, Italian gratis; Eastern Vulgar Latin: Romanian gratis

On the evolution of gratis, consider the parallel remarks of Benveniste on its development from Latin into French: "Lat. gratiosus can mean “who feels gratitude” and “held in favor, popular” and also “what shows favor, gracious.” With the same specialization, gratiis contracted to gratis, which French has borrowed from Latin, means “without paying”: gratis habitare ‘to live for nothing, without paying rent’. In this way there appears in the use of gratia a new sense, that of a service provided or obtained “by grace and favor, to give pleasure.”" ~ E. Benveniste, Indo-European Language and Society (1973)