The GUG start codon in E. coli: identity of initiating tRNA and efficiency of translation
The NCBI translation table translates all alternative start sites as methionines. To my understanding, all translation is initiated by the fMet-tRNA. I don't know if there are any exceptions to this rule. 

Regarding translation efficiency, I only found a 1985 paper in PNAS (Reddy et al, PNAS 82:5656-60), in which they compared the translation efficiency of adenilate cyclase's own UUG start vs GUG or AUG, obtaining a translation ratio 1:2:6 UUG:GUG:AUG, suggesting that AUG is the most efficient one, followed by GUG. Also, Romero and Garcia, FEMS microbiology letters 84:325-330 (1991) compared the efficiency of AUG vs AUC, AUA and AUU, showing a much lower efficiency for those codons, but they did not compare it to GUG.