After Salomon Rondon put West Brom ahead, Leicester hit back through Danny Drinkwater and Andy King, only for Gardner to earn the visitors a point by arcing a free-kick into the top-left corner in the 50th minute.
Tottenham Hotspur can go above Leicester on goal difference if they win at West Ham United on Wednesday, while Arsenal will be just three points behind them if they overcome Swansea City.
“We made a lot of chances, but the ball didn’t want to go inside. This is football,” Leicester manager Claudio Ranieri told Sky Sports.
“But I’m more happy now than after the match against Norwich. Tonight we played much, much, much better.
“We believe in ourselves and we continue to fight. Until the end, every match is important to us.”
Ranieri made two changes to the team that had edged Norwich City 1-0 at the weekend, Danny Simpson and King replacing Daniel Amartey and the injured N’Golo Kante, and saw his side fall behind in the 11th minute.
Darren Fletcher’s through-ball released Rondon and the Venezuelan striker shrugged Robert Huth to the ground before steering a shot beneath Leicester goalkeeper Kasper Schmeichel.
Drinkwater equalised on the half-hour with a shot that deflected off Jonas Olsson and looped over Ben Foster and after Jamie Vardy had headed against the bar, Leicester went ahead in first-half stoppage time.
Marc Albrighton’s crossfield pass from the left was audaciously back-heeled into King’s path by Riyad Mahrez and the Welsh midfielder swept home.
But five minutes into the second half Gardner levelled after Mahrez had been penalised for handball, caressing the ball over the wall and past a flat-footed Schmeichel from around 22 yards.
Leicester piled on the pressure in the latter stages, Shinji Okazaki heading against the bar and Foster thwarting substitute Jeff Schlupp, Vardy and Wes Morgan, but they could not find a way through.
Chelsea extended their unbeaten run to 12 league games with a 2-1 win at Norwich City that saw them climb from 11th place to eighth, five points below the European berths.
Kenedy put the visitors ahead after just 39 seconds, the Brazilian wide man drilling home left-footed from outside the box to register the fastest goal of the season to date.
Bertrand Traore freed Diego Costa to lob Norwich goalkeeper John Ruddy from what looked like an offside position in first-half stoppage time, with Nathan Redmond reducing the arrears in the 68th minute.
“We didn’t want to go into a battle, but they forced us into a battle,” said Chelsea interim manager Guus Hiddink.
“We could have finished it off earlier in the first half. We were a little complacent. At the end they made our life very difficult.
“Everyone knows how we started in December. We were almost in the relegation zone. Now we’re winning and that’s encouraging.”
Fabio Borini struck a stunning last-minute equaliser to earn Sunderland a 2-2 draw at home to Crystal Palace that lifted Sam Allardyce’s side out of the relegation zone.
After Dame N’Doye’s deflected shot put Sunderland ahead, Connor Wickham struck twice against his former club to put Palace in front, only for Borini to smash home from a seemingly impossible angle at the death.
Everton ushered bottom club Aston Villa a step closer to relegation with a 3-1 win at Villa Park that left Remi Garde’s side eight points from safety.
Goals from Ramiro Funes Mori, Aaron Lennon and Romelu Lukaku put Everton in control, before Rudy Gestede headed in a 79th-minute consolation.
Meanwhile, seventh-placed Southampton squandered an opportunity to close on the teams above them in a 2-0 loss at south-coast rivals Bournemouth, who prevailed through goals in each half from Steve Cook and Benik Afobe.