The authors agree that the surname is of Germanic origin. V. Blunda describes it as derived from the Germanic compound wil ("will, desire") and hard ("hard, valiant"), with the meaning "strong in will" or "firm in purpose", noting that the root wil is the same one that gave modern German Wille and English will. Surnames in Italy proposes instead the base Gisilhard, with the sense "strong with the dart", linking it to the Val Seriana.

E. Rossoni suggests that all the variants could derive from the medieval French name of the Languedoc Gailhard, transformed in the medieval period into Gilardus, documented in a text of 1305. According to Rossoni, Ghilardi shows isolated pockets in the Lucca area and in the region of Lombardy. Gilardi, by contrast, is widespread throughout north-western Italy, with a core between central-western Lombardy and Piedmont, and a presence also in the province of Rome and in northern Campania.

Rossoni further documents several derived forms: Gilardini and Gilardoni are proper to the Lombardy area; Gilardino, to the Piedmont area; Gilardo, very rare, also to Piedmont, with a presence in the Savona area; Gilardone, to the Alessandria area; and Gillardo, which would appear to be typical of Pareto. Blunda, for his part, notes that the spelling with an intercalated -h-, characteristic of northern Italy, reflects the guttural pronunciation of that consonant in intervocalic position, a feature that distinguishes the northern forms from the southern ones.