Bob, a tomcat, who got lost

[Bassim]

I am wondering if my sentence is grammatically correct.

Bob, a tomcat, who got lost during a heavy flood, has been found and united with the Thompsons a month later.

 

[J&K Tutoring]

Almost.

Bob, a tomcat who got lost during a heavy flood (no comma after tomcat), [STRIKE]has been[/STRIKE] was found and reunited with the Thompsons a month later.

Has been found, a preset perfect form, doesn't work with the past setting "a month later". This knowledge of the sequence of events (and the passage of time) demonstrates that it all happened in the past, so past tense forms are required:

Either had been found and reunited or was found and reunited.

If we place the reuniting in the present, then:
Bob, a tomcat who got lost during the heavy flood a month ago, has been found and reunited with the Thompsons.

 

[Tdol]

You could say who had got lost, though it's not necessary.