If you drove the car while it was overheating due to the pump failure then yes, this could have caused the head gasket to fail again.
On the other hand it is possible the mechanic fitted a sub-standard head gasket last time, or did a poor job of fitting it.
It all comes down to whether you trust him to do a decent job at a fair price or if you would feel happier taking it elsewhere.
Either way, if the sniff test is detecting exhaust gases in the coolant the head gasket will need replaced.
I'd recommend you get the cylinder head checked for flatness while it's off, it is probably warped and in need of a skim.
edward t
October 2009