The level 3 RNA-seq data from TCGA provides gene expression in terms of FPKM. From what I know RPKM is for single end RNA-seq, while FPKM is for paired end RNA-seq.
Check this link : http://www.rna-seqblog.com/rpkm-fpkm-and-tpm-clearly-explained/
Better to use the provided expression metric, unless you intend to secure raw data from TCGA and process from the start.