The 38-year-old impressed for the Tigers in their draw with Wealdstone on Saturday, keeping a clean sheet in the process.
Complications with loan signings meant Mike Cook needed a stopper and King admits he initially thought someone was pulling his leg.
"I had a call from Hosky (Andy Hoskins) at about 6pm on Friday night," he told Gloucestershire Live.
"I'd just come in from work and was having dinner with my family, so I missed the call.
"He tried again so thought something was up and he started by saying 'how do you feel about making your National League debut tomorrow?' - I had to check it wasn't April the first!"
A modest King also praised the team's defence who he claimed helped him a lot.
"I didn't have a massive amount to do," he said. "The defence have been brilliant recently and they and the whole team put in a real shift today and made my afternoon a bit easier."
He also impressed manager Mike Cook who said he knew he could count on King.
"He's always been a stand-out goalkeeper and always reliable," he told the club's Facebook page.
"He did really well and I'm really pleased for him."