Are full bright texturepacks allowed? if not can I use a mod or set my gamma really high?

texture packs: no. read the rules, you may be allowed to change the gamma value

Where are the rules?

https://www.speedrun.com/mc?h=Any_Glitchless-random-seed-1.16&rules=game&x=mkeyl926-r8rg67rn.21d4zvp1-wl33kewl.4qye4731 u can change ur brightness up to 500% in the in game options if u use the speedrunning sodium