The final scoreline flattered the Pride, who struggled with cohesion in attack and defence as they were outscored six tries to four.
Inaccurate goal kicking threatened to thwart the Gurias when the Pride drew within six points after late converted tries to Sam Pau and Aidan Smith. But Gurias halfback and man of the match Ase Boas, who finished the game with two tries, sealed the victory with a field goal near full-time.
After watching the Pride beat the Cowboys under-20s 20-18 the previous week, Demetriou was disappointed his team didn't aim up against the Gurias.
"I felt the last week or so the attitude hasn't been great at training," he said.
"I addressed that last night (Friday), but I think that was the case today.
"I don't think we muscled up. We came up against a very physical and good side.
"The Gurias are really well drilled and came at us with some good firepower and we didn't match it early on.
"In the second half we did a lot better. We broke down technically, but physically we aimed up a lot better."
The Pride dominated field position in the opening stages and found themselves 6-0 in front in the fifth minute following a converted try to fullback Hezron Murgha.
The home side appeared certain to cross again in the next couple of sets but were denied by the Gurias' scrambling defence.
The visitors then hit back on their first visit to the other end when Boas found space after the Pride defence rushed up on the left edge.
Boas was in everything and scored again later in the half along with Breadly Simon to set-up a 14-6 advantage for the Gurias at the break.
Demetriou, who took a squad of 27 into the game, made a host of changes for the second half, and the Pride's fortunes looked brighter when winger Davin Crampton scored courtesy of a grubber kick two minutes after the restart.
But the Gurias quickly regained control of the match as they began finding gaping holes in the Pride's backpedalling defence.
Nelson Daplen, Albert Patak and Dion Aiye all scored in quick succession to establish a match-winning lead.
The Pride play their final trial match against Mackay in Ayr on Saturday, with Demetriou planning to field a full-strength side.
"I had to know who's up for it and who's not, and who's good enough and who's not at this point in time," Demetriou said.
"I learnt a lot today from within my whole squad."