A lady I was starting to enter the gate with was stopped by me and the gatekeeper.
Literal
Gate [from-から] enter-[half-way-かけていた] lady [and-と] that gatekeeper [by-に] was-stopped.
This sentence is oddly structured — the intended meaning is probably 'The lady who was about to enter the gate was stopped by the gatekeeper.' The particles don't quite work in a standard reading; とその門番に could be read as 'and I was stopped by the gatekeeper along with the lady.' ~かけていた is the past progressive of ~かける ('begin to but not complete'). The ambiguity is real and a learner can be forgiven for finding it confusing.