This fragment continues onto the next page (inc.: -guas me neque in ira tua... [Ps. 6.2]) in the manuscript, though it was not previously apparent in the digital presentation.
The text of the fragment, strangely enough, is the same as the main text of the manuscript, namely the abbreviated Psalter compiled by 'Einhard'. It appears in full (including the sections found in the fragment) from pp. 3-43. This abbreviation is based on the Gallican psalter (iuxta LXX).
This suggests that St Gall may have had two copies of the Einhard abbreviated psalter, one of which was later used to bind the other.
The edition of Einhard's abbreviated psalter in CCCM 47 (ed. P. Salmon) is based on a manuscript now in Vercelli (A.C. ms. 149). That manuscript's text actually uses the Psalterium romanum (online here) as its basis, though it is identical in terms of the preface and the verses selected to the main text of CSG 97.