Put the -c option just before its argument – the query. Mind also using the additional -t option to get just the tuple value. And of course, use the backticks (`) operator.
Using the -X option is also recommended, as sometimes a .psqlrc file might add some redundant output, as well as the -A option, which disables column aligning (whitespaces).
In order to skip NOTICE or other additional messages, include the -q flag.
vartest=`psql -d $db -U $user -AXqtc "SELECT gid FROM testtable WHERE aid='1'"`