Will the 99s be used on London Gateway freights?
I don't know for certain. GBRf, who have ordered the 99s, seem to have around half a dozen freights a day out of Gateway to places like Hams Hall, Birch Coppice, Doncaster, Masborough. Last Thursday these all went on electrified routes, at least as far as Rugby or Peterborough. So I would say it is pretty likely, at least for those routes that are pretty flat once you get beyond the wires.
Other freight movements out of Gateway seem to by Freightliner (around a dozen a day) and DB Cargo (two/day). If the 99s are as competitive as they say on operating costs against 66s, then I would expect GBRf to be bidding strongly for some of these flows from Gateway - to places like Trafford Park and Coatbridge, which are long routes nearly all under the wires.