You are not going to ruin anything 'cg' by shampooing before colouring.
Permanent colour and bleach is a zillion times more damaging than one application of shampoo. Semi permanent colours often have surfectants built right in, so whether you like it or not, colouring your hair is going to get stripped from it's natural oils.
It depends on the type of colour you're using as to whether it will be affected much by the gel. Also depends on how much gel. I would tend to wash it first, but if using permanent colour and not much gel, then going ahead without shampooing is probably just fine.