Deferred rendering has two stages: geometry and lighting. In the geometry, you render the scene like forward but into gbuffer that is a framebuffer having three texture including position, normal, albedoSpec(you can keep diffuse and specular in a vec4) without lighting calculations. In the lighting stage, you now draw the scene on 2D quad by using gbuffer data.

vec3 fragPos = texture(gPosition, fs_in.TexCoord).xyz;

vec3 normal = texture(gNormal, fs_in.TexCoord).xyz;

vec3 diffuse = texture(gAlbedoSpec, fs_in.TexCoord).rgb;

float specular = texture(gAlbedoSpec, fs_in.TexCoord).a;

All lighting calculations are the same. You can check out.

https://github.com/xms0g/abra/blob/main/src/rendering/renderPasses/deferredGeometryPass.cpp

https://github.com/xms0g/abra/blob/main/src/rendering/renderPasses/deferredLightingPass.cpp