Hammergel does contain caffeine in some of the flavours... espresso, for one (I think its 50mg/serving). You could try grinding up pills and mixing it in, but I have a feeling that would be pretty nasty.
Personally I like espresso Hammergel, and "just plain" GU - when it's really hot out, I can't take the syrupy sweetness and intense flavour of the espresso.
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