Spurs deservedly led through Sebastien Bassong's first goal for 16 months but Gardner salvaged a point 10 minutes from time.
Liam Ridgewell was yellow-carded after 18 minutes for bringing down Gareth Bale and, from the resulting free-kick, Spurs went ahead.
Ben Foster got a touch on Bale's inswinging free-kick, but Bassong was first to react and hammered home.
Blues were looking vulnerable and another dangerous cross from Aaron Lennon picked out Bale, but he failed to make proper contact with his volley.
However, Birmingham mounted a concerted spell of pressure after the break and when Roger Johnson's deep free-kick was headed back across goal, Gardner timed his run perfectly to head his fifth goal of the campaign.
Source: PA
Source: PA