The name gerðr is usually traced back to garðr, which means enclosure or the fence itself. The common interpretation is therefore that She’s a giantess of the fenced land, presumably farmed or grazing soil. This would go well with Frey and give their union the sense of the wild fields being cultivated to produce crops – the fertility-giving god plows the giantess of the land. This sort of sacred marriage is sometimes hinted at in skaldic poetry from the late tenth century, though the god in those cases is Odin and the giantess is Jord. Whether Frey’s relation with Gerðr was ever perceived as a hieros gamos in the pre-Christian period is uncertain and the late date of För Skírnis makes it harder to distinguish the old material from medieval innovations. Yet it makes sense given Her name and His fertile nature and even if the ancient heathens never phrased the whole affair in the courtly terms used by the author of the eddic poem. To a large extent, this is one of those cases where modern interpretation and (shared) UPG fill in the gap created by a lack of pre-Christian sources.
Personally, I have little contact with Gerðr, since I’m not a farmer and I live in an apartment building. The closest I get from working with an enclosed plot of land are my mother´s potted plants, which in a way is not entirely out of place and can be a way of reaching out to Her. If Her name means “fenced land”, then the content of a vase is a close variation of it; if it stands for the fence itself, a pot can fit the description in an urban fashion. Enclosed land or the protection of it seems to be Gerðr’s dominion. She may even have been a guardian that needed to be wooed before the land could be cultivated, much like local spirits had to be placated before an area could be cleared of trees. Or She was originally a giantess that became a protector of Frey’s golden fields. This is just me speculating, but the basic assumption is, in any case, that She stands for the land and Him for the phallus.
It is sometimes said by modern polytheists that Their marriage was an act of friðr, in that it bridged the gap between the world of giants and that of the gods. Academically, it has been pointed out that while the Aesir marry other deities, the Vanir tend to marry giantess, as if there was a hierarchy (hostages yes, mingling no) or, alternatively, as if the latter were somehow closer to the giants. It is unclear to me which answer is likelier, though I get the sense that there’s a bit of truth in all of the possibilities. I really need to have a long conversation with a devotee of Gerðr!