Breakdown of Tko je ovaj gospodin?
biti
to be (sam, si, je)
tko
who
ovaj
this (m) / ova (f), ovo (n)
gospodin
gentleman
Questions & Answers about Tko je ovaj gospodin?
Why does je come immediately after Tko?
je is a short, unstressed word (an enclitic) that loves the second position in a sentence. Since Tko is the first word, je naturally falls right into the second spot to link the question together.
Why is the word for "this" ovaj here instead of ova or ovo?
Can gospodin be used as a title like "Mr.", or does it just mean "gentleman"?
It means both! In this sentence it means "gentleman". However, it is also the exact word you use for "Mr." before a man's last name, such as gospodin Horvat (Mr. Horvat).
How would I change this to ask "What is this?" instead?
You would replace Tko (Who) with Što (What). The whole question would become Što je ovo?. Notice that "this" changes to the neutral ovo because you aren't pointing at a specific masculine or feminine person anymore.