Centre-half Yakubu powered in with a header from six yards on the stroke of half-time as the Bees took an undeserved lead into the break.
And eight minutes into the second-half, full-back Gross somehow found himself clear in the Mansfield box and stroked home in style.
Mansfield, despite four successive away defeats in which they had failed to score a single goal, looked much the more confident in the early stages.
Indeed, they almost went ahead inside the opening 60 seconds when Matt Hamshaw found himself in the clear from Chris Beardsley's flick-on.
Having cut inside, his curling effort beat keeper Lee Harrison, but unfortunately for the visitors then smacked against the post and rebounded to safety.
On the quarter-hour mark, it was Beardsley's turn to shoot at goal after a smart turn. This time Harrison was equal to the effort and he produced a decent save.
Barnet looked totally devoid of ideas going forward, no doubt disrupted by a striker crisis.
With Adam Birchall, on loan from Mansfield, not allowed to play, Liam Hatch suspended, Tresor Kandol on loan at Leeds and Giuliano Grazioli and Jason Norville injured, they were left with just Magno Vieira as a recognised striker.
Paul Warhurst was therefore pushed up front, but he hardly had a sniff as the Bees failed to grab a hold of the game.
They continued to ride their luck at the back, with Harrison again forced into a brilliant save to keep out a powerful header from John Mullins who was left unmarked in the middle from a corner.
Cruelly for Mansfield, though, by the time blew for half-time, they found themselves one down.
Given Barnet's lethargy up front it was perhaps no surprise their goal came from a set-piece, with Yakubu rising highest to connect with Sinclair's corner and head into the net.
The undoubted lift carried through into the second-half as the Bees came out much better.
And it was not long before Warhurst's neat flick released Gross beyond the last defender and he kept his nerve to convert into the corner.
Mansfield finally broke their away duck with just five minutes to go when Richie Barker headed in an inswinging free-kick, but it was too little, too late as the Stags failed to complete a dramatic comeback.