When making my first ever batch of red velvet cake, I tried using beet juice in place of food gel. It didn't work AT ALL. I actually used so much beet juice to try to produce the red colour I was after that it completely ruined the taste and texture of the cake. Plus the colour only really stayed on the outside- the inside didn't have even a hint of red in it. I have heard of other people who have had great success using ground, dried beet powder in place of red food gel. I can't comment on the results as I haven't tried it myself but I'm going to take a guess and say that the food gel will produce a more vibrant result than beets will.