Rangers slipped 12 points behind Ladbrokes Premiership leaders Celtic after making it 10 points dropped in 2020 – eight from a winning position.
Gerrard's side have conceded six goals in their last three away games in the league, taking only one point in the process.
The Rangers back-four looked ragged from the start in Perth and conceded inside eight minutes when Callum Hendry finished well after Niko Katic failed to deal with a high ball.
Half-time substitute Florian Kamberi inspired a comeback, netting a brilliant volley and setting up Joe Aribo to net his second goal this week from an unfamiliar left-back role.
But Rangers conceded a corner following another long ball and the delivery ultimately fell for Stevie May to drill home from eight yards in the 80th minute.
Gerrard said: "I am going to play the same record for you. I should be sitting here waxing lyrical about Flo Kamberi and Joe Aribo, and maybe Alfredo (Morelos) if he took his chance. From an attacking point of view, second half I thought we were outstanding.
"But we keep shooting ourselves in the foot by conceding silly, silly goals and we forgot the real good, strong basics of defending. It has cost us on two occasions, and nearly three, only for Allan McGregor to pull off a world-class save.
"I'm frustrated and concerned with us defensively because it has happened too many times now. That is Aberdeen, Killie, Braga, Hearts, St Johnstone. That is too many times for this football club. I am going to have to find solutions."
The Rangers manager agreed there was a nervousness about his defence.
"Basically just not doing the basics of defending well enough – that is what it is down to," Gerrard added.
"Flo's first goal is a world-class finish and then with a bit of skill on the side, he gets the assist and completely flips the game on his own with individual brilliance.
"Again, Joe Aribo stands up and is big for the team to get us in front and then we have an incredible chance to kill the game off.
"Alfredo misses that and then, because we haven't killed the game off, you are hoping from the side there is enough experience and quality to see the game out, but again the basics of defending we get wrong.
"A straight ball over the top, confusion and we actually get away with that but give a set-piece away and don't defend the set-piece well enough, which is just a simple cross into the box that we don't defend. It is really frustrating."
St Johnstone boss Tommy Wright started with Hendry and May up front and encouraged both of them to run on to long balls down the sides.
"We took a bit of a gamble," he said. "It leaves you short in midfield but we worked hard on trying to be narrow and compact.
"When you play two up, it is difficult. A lot of centre-halves nowadays are not used to it, they are used to just one striker to deal with.
"No defender likes to be turned and running back towards their own goal.
"The way Rangers and a lot of the top teams play, by pressing their full-backs high, they are often prepared to leave two versus two. It was something we thought we could get a bit of joy off and pleased that that happened."
May should have been off the park five minutes before he scored – Wright changed his initial thoughts about taking Anthony Ralston off for Michael O'Halloran and decided to bring his striker off.
But the change of plan did not get through to his kit man, who held up the right-back's number.
Wright added: "When you put that much into a game, you deserve a bit of luck."