However, a try from replacement hooker Vili Ma'asi made things interesting and kept alive the possibility of an unusual home reverse for the Cherry and Whites.
Gloucester seemed to be in control but simply couldn't do anything with good possession and territory in the final stages when any score would have put the game to bed.
Credit to London Welsh who fought doggedly throughout. They had already enjoyed a competitive run out last weekend and it showed.
They did walk a tightrope in disciplinary terms and certainly got under Gloucester's skin. But they never threw in the towel and their performance suggests that they will be a force to be reckoned with in the Championship this season.
For Gloucester, the defeat will serve as a wake up call and some hard work on the training ground and in the analysis suite will no doubt lie in wait this week ahead of the Russia game on Friday.
But, nothing is won or lost in preseason and this performance will certainly gives the coaches food for thought.
The opening minutes were most noteworthy for a late tackle on Freddie Burns which left the fly half needing treatment but Burns dusted himself off and opened the scoring with a 7th minute penalty.
The visitors continued to be physical. Scrum half Nick Runciman was facing his former club but wasn't receiving any preferential treatment and had to leave the pitch with a blood injury to be replaced by Dave Lewis.
The Exiles were certainly up for the game and scored the game's first try on 13 minutes. Number eight Ed Jackson burst through a gap after a good spell of possession before offloading to lock forward Mike Powell who cantered over. Alex Davies added the conversion.
Things were simmering amongst the two forward packs and spilled over as Gloucester drove for the line. Will James and Exiles skipper Jonathan Mills swapped a flurry of punches and were both duly yellow carded. Gloucester pressed but had to be content with a 20th minute penalty from Burns.
Burns nudged Gloucester in front 9-7 after Tom Voyce lined up his opposite number Will Hurrell to force a turnover as the half hour approached but there was still an undercurrent of niggle present and both skippers were lectured by the referee.
Welsh fly half Davies restored his side's lead at 9-10 with a penalty when Gloucester were penalised at a ruck head on to the posts as the visitors continued to frustrate Bryan Redpath's side.
However, going back to basics did the trick as the pack drove the visitors back before Mills almost smashed his way over on the burst before Trinder repeated the trick and just made the line. Burns added the conversion to give Gloucester a 16-10 half time lead.
Gloucester deserved their lead but were being given a real work out, quite literally, by the visitors.
Aside from the yellow cards, Burns had been taken out late on three occasions and various off the ball incidents were threatening to take the gloss off the game.
Gloucester started the second period with purpose but the precision was still a little lacking and Burns had to pull off a stunning cover tackle to deny the visitors after an interception from deep.
A quick tap from Dave Lewis gave Gloucester their first real go forward possession and Will James charged through the middle to make 30 quick metres with Peter Buxton in support. Trinder took it on but couldn't free his arms with an unmarked Jonny May outside him.
The try wasn't long in coming though. Gloucester kicked to the corner, set the maul and drove the visitors back over their own line. Darren Dawidiuk claimed the try and Freddie Burns converted.
23-10 looked a decisive lead but London Welsh hit back with a somewhat soft score as replacement hooker Vili Ma'asi spun off a mail on the Gloucester 22 and tiptoed his way down the 5m channel to score in the corner. Former Scotland international Gordon Ross converted from the touchline.
The final quarter was disjointed as neither side could retain sufficient possession to severely challenge the other's defence and numerous replacements had the obvious impact upon the flow of the game.
That said, Gloucester will kick themselves for not putting this one to bed and, as time ticked away, so the feeling grew that the visitors could nick it.
And, so it proved, as the game went into injury time, a half break and an offload opened up the Gloucester defence and scrum half Rob Lewis dived gleefully over. Ross converted for 23-24 and the Exiles had a famous win in the bag.