The 25-year-old only switched codes last season but Gustard has seen enough so far to suggest the former Wigan Warrior will go a long way in the 15-man version of the sport.
"I am expecting big things from centre Joel Tomkins this season," Gustard told the Rugby Paper.
"We desperately want him to be a success in union not just for Saracens but for England as well. He is good pals with Chris Ashton and I think he has high expectations of himself.
"We made a deliberate point of signing him midway through last season so he would have a few months to acclimatise to the demands of union.
"It means he comes into pre-season fully understanding all the intricacies of the breakdown and the other regulations so he is, in effect, on a par with every other player.
"He is 6'3, 15st 7lbs, is lightning quick and is a brilliant offloader - that is a skillset for an outside centre that would benefit any international side.
"You obviously have Manu Tuilagi who is a very different beast, but I think Joel could be a huge success for England."
If Tomkins does impress this term and force his way into the reckoning for selection for Britain and Ireland's elite next summer, he would join a stellar list of rugby league converts to have worn the famous red jersey.
Fellow Wigan star Jason Robinson is the most recent rugby league name to star for the Lions, touring twice in the noughties and scoring five tries on his Lions debut 11 years ago.
In 1997, Sir Ian McGeechan selected a whole host of recent converts, although Alan Bateman, John Bentley, Scott Gibbs, Scott Quinnell, Alan Tait and Dai Young had all played rugby union prior to their stints in league.