Genes can have multiple transcripts. So gene expression could mean the overal expression of all transcripts of a gene. Transcript expression is the expression of a specific transcript. In the past microarray period gene expression was measured and the output was gene expression. Nowadays with next gen sequencing methods like RNA-seq, CAGE etc. specific transcript expression can be identified... Maybe read a bit about alternative splicing (http://en.wikipedia.org/wiki/Alternative_splicing)
In a sense, there is no such thing as a gene. A gene is a concept: a fuzzy bounded region of a chromosome within which transcription occurs. So really there is no gene expression, only transcript expression.
I'd even argue that "expression" itself is not a very useful term and we should simply use "transcription".
It may also depend on the tools you're using. You may be able to determine that there is expression from a particular gene, but the tool you're using may not be able to differentiate between alternative splicing - in this case I would talk about gene expression. If I had more accurate tools, however, I might talk about transcript expression.
Eg, an RT-PCR using primers that both fall in the middle of a large constituent exon would give us info on gene expression. An RT-PCR using primers that spanned an intron junction that was specific one splice variant would give us info on transcript expression.