ALIA from GEDCOM 5 is a link to another individual.
There is no tag ALIA:GIVN.
The "normal" way to add an alternate name is to create a new NAME structure
Basically, you could then use : <prop path=INDI:NAME?1:GIVN>
Another way to do it is to add a nickname (tag NICK).
If your alias is indeed a link to another individual, you can try <prop path=INDI:ALIA:*:..:NAME:GIVN>
Zurga