The argument for the possible location on Garway Hill runs thus. Licat Amir is on the Gamber somewhere (see Note on the name). Bruce Coplestone-Crow has suggested, not unreasonably, that the name mutated into Lagademar* ("Lagad-emar"), which is is recorded in the Domesday book as in Archenfield, TRE* (that is, tempora regis Eduardis - in the time of Edward the Confessor). Lagademar is then annotated "Garwi" in the "Herefordshire Domesday" book* (a 1160's copy of the Domesday with annotations and updates: Info), indicating they are the same location or one is in the other.
As secondary evidence* the Domesday Book notes the current holder of the land as Herman* (de Dreux*). In 1189, Richard I entitled to the 'Knights Templar' the area of Langarewi (i.e. Llan Garewi; Llan Garway) including with it castellario quod frit Hermanii*. Coplestone-Crow has suggested the castle was at Welsh Newton, where Pembridge Castle stood - this would (apparently) have been within the manor of Garway* (Castlefield Farm, just south of Garway Hill, stretches towards Grosmont Castle on the first Ordnance Survey maps, while the castle north at Orcop isn't mentioned in the Domesday book and is therefore thought to be later - neither are therefore likely to be associated with Herman's castle).
As for the exact spring, there are a variety of sources coming from the east of Garway Hill and eastern Garway (See Visit Details for examples), all of which enter the Garre(/a)n River*. The 1st Edition 1:2500 (1854-1949) O.S. map of the area is littered with springs and spring-related names (e.g. Garway Hill's Spout Cottage and Hawthorn Well; with possible springs at Glasshouse Farm, north of Trothland; or on Garway Hill at Well House, Garway Hill Chapel, or below the earthworks at Suckling Dingle - note that likely candidates don't include the spring at Garway Church, which almost certainly ran west into the Monnow [Details]). Taylor's map of 1754 shows a Garren Spring on Garraway Hill, which was presumably regarded as the source of the Garron.
Sadly, having potentially identified the rough area of the Licat Amir Coplestone-Crow had to leave it at that as, in 1986, there were no known burial mounds in the area. However, in 1993 a potential long barrow was identified near the TV mast* on Garway Hill (long barrows are usually Neolithic : Info). The location of the barrow is discussed in this Note and photos can be seen in Visit Details. However, in addition there are a variety of earthworks in the area (Details) which might be of any age up to the early medieval. Garway Hill itself is covered in earthworks and it is difficult to say what is or is not ancient (See Visit Details).
It should be noted that the nearby Coles Tump is a natural feature, though the reason for its name is unclear and ring and mound features on the summit, while suspected of being mediaeval, await definitive dating [see Note]).
This does leave the problem of the name change. Coplestone-Crow notes the complete swamping of the original name into "Landmore" in a 1505 reference*. A later reference to this can be found in a 1585 Survey of the Manor...
One messuage and lands called llandmore in Garway; formerly demised to Thomas ap Ieuan, now John ap Ieuan, 10s.
(Rentab and Surveys, Heref.27 Eliz (1585) Misc. Bks. vol 217 ff.5-7 Garway, Survey of the Manor latin, Condensed*)
However, this parcel of land is lost from any currently available maps.
Given the absence of any mention of Garway in the Historia wonder description, it is perhaps worth considering the name "Garway" more generally. Its absence does not necessarily dictate against the wonder being in the area - the name is also missing from the Domesday, possibly suggesting it wasn't much used. It certainly appears to be a name at least as old as the Historia, so that is no reason for its exclusion. The oldest part of the Liber Llan Dâv (See Other Works) gives the name as Llan Gurboe / Gu(o)ruoe (where u would later be replaced by v or w). The book also suggests the area was given by Guorvodu, king of Ergyng, to Bishop Uvelbin, who gave its living to his priest, Guorvoe, hence the name. The use of the double-L here suggests a later date for this text, however, the book also gives the second grant of the land of lann Gouruoe as from Athruis, king of Gwent, to Bishop Comegerius to secure the soul of the king's father Mouric (Meurig) - a grant witnessed by the abbot of Lann Guruoe. This event is most likely to be dated to the 6th C.*, and this spelling may be closer to this than the above variant. The grant is interesting if we consider the oft-made suggestion that Arthur and Athruis might be identical, though the evidence for this is thin. This grant included Wormelow as well, as part of the parish of Dewchurch Magna.
White has suggested the name "Garway" alternatively derives from Gaergwy ("fort above the river")*, while Mathews* suggests it may also derive from a discription of the river Garron as garw wy "the rough stream(?)". Whichever is true, all the above strongly suggest a pre-Saxon influence - confusing in light of the name's absence from the Domesday Book. Strangely, given this set of potential origins, there is an alternative Welsh name for Garway of Llanwrfwy*. This simply seems to have been derived from an earlier spelling ((G)uorvei?)(note also that in the local village name Bagwy Llydiart, Llidiart refers to a "gate" and is not a corruption of Llicat).
Incidentally, just in case you're looking to plot the next in a long line of dubious Templar books, you might be interested to know they built one of their round churches in Garway (See Visit Details: Plan). Even with rebuilding, it is an unusual and beautiful piece of history (Info; Info). There are no burial mounds in the vicinity - despite leaflets in the church indicating ancient earthworks in the field below the building, however, it's not uncommon for churches to be built on the site of previous features (henges, for example) so it is possible a tump has been destroyed. One can but look forward to the suggestion the church was built on the site of our wonderous tomb...