The only free and comprehensive online etymological dictionary of the Spanish language
grave (Adjective) "heavy," "serious"

10th cent. From Latin gravis 'id.' From Proto-Italic *grawus 'id.' From Proto-Indo-European *gwreh2-u̯s 'id.' From the root *gwreh2- "to be heavy."

Romance: Western Vulgar Latin: Portuguese grave, Catalan greu, French grave, Italian grave; Eastern Vulgar Latin: Aromanian greu, Romanian grev

Italic: Oscan bravús

Indo-European: Germanic: Gothic *kaurus; Hellenic: Ancient Greek βαρύς (barys) "heavy" (whence bajo); Indo-Iranian: Sanskrit gurú-; Tocharian: A krāmärts, B krāmär "weight"
grupo m. (Noun) "group"

15th cent. meaning "group of clouds." The modern sense is first recorded in the 18th cent. Borrowed from Italian gruppo 'id.,' itself borrowed from Old Occitan grupo, which in turn was borrowed from a Germanic source. Ultimately from Proto-Germanic *kruppa- "body (of things)." Derived from Proto-Indo-European *gru̯bh-néh2- "to crawl" from a sense of crumpling up.

Germanic: North Germanic: Old Norse kroppr "body;" West Germanic: Old High German kropf "goiter," Old English crop "sprout" (English crop)
guacamole m. (Noun) "avocado-based salad," "avocado condiment"

Very late 19th cent. From Nahuatl ahuacaoulli 'id.,' comprised of ahuacatl "avocado" and molli "sauce." See aguacate and mole (3) respectively.
guardia f. (Noun) "guard"

12th cent. Borrowed from Gothic *wardja "watchman." Derived from the verb in Proto-Germanic *warjan- "to prevent." From Proto-Indo-European *h2u̯or-éi̯e- 'id.'

Also the origin of the surnames Guardia, Guarda, La Guardia, Laguardia, and Guardias.

Germanic: East Germanic: Gothic warjan; North Germanic: Old Norse verja; West Germanic: Old Saxon werian, Old High German werren, Old English werian

Indo-European: Balto-Slavic: Lithuanian vérti "to pierce," Latvian vẽrt "to open;" Hellenic: Ancient Greek ἀείρειν (aeírein) "to raise;" Indo-Iranian: Sanskrit vr̥ṇóti "to cover," Young Avestan vərənaoti 'id.'
guay (Interjection) "woe!;" (Spain) "great!"

13th cent. From Gothic wai "woe." From Proto-Germanic *wai 'id.' From Proto-Indo-European *u̯ai̯ 'id.'

The Spanish colloquialism is a secondary formation through meaning reversal.

Italic: Latin v

Germanic: North Germanic: Old Norse vei; West Germanic: Old High German , Old Saxon , Old English (English woe)

Indo-European: Celtic: Old Irish fáe, Middle Welsh gwae, Middle Breton goa; Balto-Slavic: Latvian vai; Armenian: vay; Indo-Iranian: Avestan vaii-ōi
guerra f. (Noun) "war"

11th cent. Disputed etymology. Certainly from a Germanic source. Following Corominas (1991) and Roberts (2014), perhaps Proto-West Germanic *werra "confusion (of battle)," or a similar meaning. From there, the traditional etymology is from Osthoff (1910) that posits an origin in Proto-Germanic *wersa- "confusion," "disruption," "greater evil." From Proto-Indo-European *u̯ers- "to confuse."

Also the origin of the surnames Guerra and Guerro.
gustar (Verb) "to please," "to like;" "to taste"

13th cent. Old Spanish gostar. From Latin gustare "to taste." From gustus "tasting" (see gusto).

Romance: Western Vulgar Latin: Portuguese gostar, Catalan gustar, French goûter, Italian gustare; Eastern Vulgar Latin: Aromanian gustu, Romanian gusta; Sardinian gustare

Over time, the meaning drifted from "to taste" to "pleasing (in taste)" to "pleasing."
gusto (Noun) "taste;" "pleasure"

Very early 15th cent. Old Spanish gosto. From Latin gustus 'id.' From Proto-Italic *gustu- 'id.' From Proto-Indo-European *ǵu̯s-tu̯- 'id.' From ǵeu̯s- "to taste."

Romance: Western Vulgar Latin: Portuguese gosto, Catalan gust, French goût, Italian gusto; Eastern Vulgar Latin: Aromanian gustu, Romanian gust; Sardinian gustu
gym m. (Noun) "gymnasium"

Borrowed from English gym, apocopation from earlier gymansium, itself from Latin gymansium 'id.' (see gimnasio.