I have a gig tonight and was wondering whether to change my strings for it. They havent been done in a while but I was wondering if I should keep the pack as spares incase one breaks tonight.

Any thoughts?
Gotta Love Rock N Roll
Ideally, you should change your strings and have some spares, but if that can't be done, I'd leave it, or if you want, you could change the bottom three (thickest) strings because they're unlikely to break.
Always keep spares at a gig, that's the most important thing. The second most important thing regarding string change is to change them at least once a month if you gig regularly. I change mine about once every two weeks. New strings don't just feel better, they also improve your tone.
If it's creative, true to your musical goal, and it sounds good, put it in the song.

1. Guitar, delay, verb, amp, and creativity.
2. Well-tuned acoustic piano and creativity.
3. A bottle of water.
No, I wouldn't change them if your gig is tonight. There's a chance they'll still be stretching, and you do not want that at a gig.

Besides that, most strings don't sound very good as soon as you put them on, you want to break them in for a day or two. I'd leave the ones you have already on on. Bring the extra pack just in case, but you'll probably be fine.

Then in the future make sure you put new strings on at least a day in advance.

All imo of course.
Spin 'round carousel when your horse isn't screwed in.

My band:
Fractured Instinct
(For fans of Death/Groove/Prog Metal)

Ibanez RGA42E
Ibanez S420
LTD H-301
Ibanez RG520
Peavey Predator USA
Douglas Grendel 725
Line 6 Pod HD500X
^ I agree with that. I would leave you old ones on this time, and have a spare pack just in case. Sometimes it does take a while to break in strings, so stretching during a gig wouldn't be a good thing.
Epiphone Les Paul Custom AP (w/ 2 Seymour Duncans)
Jackson Dx10D Dinky (w/ DiMarzio PAF Bridge)
Epihpone Hummingbird

Marshall JCM 2000 DSL 100 (Voodoo Modified)
Custom 4x12 Halfstack (w/ Veteran 30's)
My opinion. Strings are good when they are new, but at their best when theyve been played for maby an hour.

For a gig, Id bring a backup guitar AND a set of strings. Its good to take a knowledgeable friend with you, (if possible) so if a string breaks (or other shit happens), change to your backup guitar while youre nice friend replaces the string(s). After that I suggest you buy him a beer/soda to thank