Hi diski,
As I'm sure you already know, one of the problems with going to the gym after work, if you are diabetic, is that exercise uses your stores of energy, so you body tries to creat more by converting more of the sugar in your blood into energy. This comes at a time of the day when your blood sugar levels are already dropping (unless you like a very late lunch). So there is a danger of a hypo if you don't take some fuel on board.
I don't know what other regular exercisers think, but I would think you could probably eat an energy bar (like a Special-K bar or a Jordans muesli bar) with no ill effects as your workout will neutralise the sugar/carbohydrate content. If you need a drink, and bottled water doesn't appeal, then have a fruit drink like unsweetened apple juice, which is low GI so won't cause a sugar spike.