> Gestur latti (letja) þess
> Gest held-back from that

"Gest tried to dissuade him from that"
"Gest tried to talk him out if it"
"Gest spoke against it"

The direct object of 'letja' (Þangbrand, accusative) is implied. I
think the definition "hold back" in CV and Zoega is potentially
misleading; as far as I know, 'letja' only ever means "hold back" in
the sense of "to try to hold someone back from doing something", not
in the sense "hold back (oneself) from doing something" (for this it
would need to be reflexive in Old Norse: 'letjask'). CV has some
useful examples.

http://lexicon.ff.cuni.cz/png/oi_cleasbyvigfusson/b0385.png

(Note: there is a printing mistake in CV's etymology. "A.S. letjan"
should read "Ulf. letjan = CRONIZEIN, ana-latjan, ga-latjan =
EGKOPTEIN; A.S. lettan" - with the appropriate Greek letters in place
of my capitals.)