I've got gel under my bar tape but only because I like a fat bar and the bars on the Black Beast are 'standard' (whatever that means these days
). Use the gel to gain thickness - bar tape has enough give on its own to provide comfort.
Before retaping your bars, think about the brake levers - would you like a bit of extra around there? Maybe there's a hollow you'd like to fill in (my problem). Maybe you'd like a bit of padding just 'there'. Think about it now because you put that extra padding under your bar tape.
You can reuse the stuff quite often (don't ask me how I know, I just promised the siamese I wouldn't swear on the forum again ... tonight), but like everything, it reaches a point where you'd like some new stuff.
My take on it? Buy new when you feel the need. It's like a new pair of shoes - old and comfy is really nice but when it starts to get tatty or fall to bits, maybe you need some new stuff.
When you put it on, start at the bar end, get plenty inside the bar end and ram in the stopper - this is a part I get wrong all the time and I was gratified (though pithed off) to note that the mechanic at the lbs is worse at it than me.
Try to wind it as evenly as you can, but as you go around curves, this becomes an interesting compromise.
Pull it TIGHT. Tightness is your friend. This ensures an even finish provided you lay it evenly and ensures that it stays where you put it. Do it too loose and it'll move. Stretch the stuff - it takes a lot of stretching and loves it. Seal the top end with electrical tape - don't bother with the miniscule bit of sticky backed something the manufacturers provide you with, go buy some black electrical tape and use it liberally (interestingly, bar tape is the only thing in the known universe that electrical tape sticks too
Putting on bar tape is an art form, so treat it as such. You aren't going to whack on a roll of bar tape and have it perfect the first time ... or if you do manage it, the second attempt will disappoint. So accept that before hand. There's nothing magical about the stuff, not even the stuff with an adhesive strip. You can put it on again and a again, until you get it right. And if you screw it up, just go buy another packet and have another go.
It's a skill that comes with practice, and is a skill that can make or mess the image of your bike. Take the time and money to practice now, redo it as you feel fit, and you'll wind up being the local guru on bar tape. It's actually one of the most satisfying skills in bike maintenance and hence worth putting a bit of effort into (much better than learning how to clean the flamin' thing).