Gillingham youngster Jarvis had netted the winner against Wolves at the weekend, while Helguson's double at Nottingham Forest had earned the Hornets a useful victory.
But Helguson blazed over the bar in the first half with just Gills keeper Jason Brown to beat after being played through by Icelandic compatriot Brynjar Gunnarsson.
And from an almost identical position after the break, Jarvis failed to keep his nerve following a mesmeric 40-yard dribble through midfield as he also smashed his shot high into the stand.
Those were the only real opportunities for either side in a game that supplied few thrills until the final stages.
Gillingham, rooted in the relegation zone after a miserable run of seven successive defeats before that Wolves win, looked like a side short on confidence.
The home side reverted to long aimless balls upfield to the ineffective Mama Sidibe and too often gave away possession to play-off challenging Watford.
However, the Hornets failed to create much themselves with a series of long-range efforts failing to seriously trouble Brown.
And even a late rally from Ray Lewington's men could not produce a vital winner as Brown was finally made to earn his wages.
First the keeper made a superb stop from a point-blank effort by James Chambers and the ball somehow stayed out of the net as Danny Webber piled in for the rebound.
Brown also kept out a Gavin Mahon header from Neal Ardley's cross, but he and Gillingham needed a massive stroke of luck to hold on for a point.
Inside the four minutes of stoppage time Chambers was left totally unmarked just six yards from goal from Helguson's cross, but he sliced his shot horribly high and wide and then Bruce Dyer saw a looping header hit the crossbar in a thrilling final 60 seconds.