I have been looking through GURPS characters and I've been unable to find any abilities that allow one to buff their allies whether it be in a way that increases their stats, gives temporary advantages, or allows them to re-roll sixes. What I wish to ask is: are there any mechanics for a character to buff friendlies in GURPS?
You have many options, of varying degrees of realism. Not all of these will be available, as they depend on which campaign switches are turned on by the GM. Here are the more straightforward, general, and common amongst them.
Afflictions (B35) are a broad-umbrella system term an Advantage that allows a character to inflict conditions, Disadvantages and/or Advantages on others. It's a mechanical term, and can represent many different in-setting methods, such as magic, cinematic bardic inspiration, highly exotic technological enhancements etc.
Complimentary Skill Rolls (CSRs)
A CSR is a generalised rule about using one skill to aid in a use of another skill, usually in a mundane way, so long as the interaction makes sense. The helper and the main actor may be different characters, or the same one, depending on circumstances. The rule was written some years after the publication of the Basic Set of 4th Edition, and as a result is now often included in various books like Social Engineering, Action, Dungeon Fantasy, the Ten for Ten article of Pyramid etc. It's a very generic rule, and a rather simple one, but whether it is applicable really depends on the circumstances.
Visualisation with Blessing
An enhancement on Powers page 87. Allows granting a rather generic bonus to rolls just by 'thinking good thoughts' (but of course can also be built as a more specific power). Starts out modest (but by default very versatile), yet becomes a very powerful ability when combined with about 30-50 points worth of the right enhancements. Is something of a controversial build from a balance perspective, in fact.
An enhancement found on Powers page 107. Allows you to share an Advantage you have with others.
Magic Magic [sic] and Other Magical Systems
The 'standard' form of magic holds many spells that grant some sort of bonus, Advantage, or perhaps some other benefit. Even more obscurely, other forms of magic, such as the standalone Ritual Path Magic (not to be confused to the magic of Ritual Paths found in the Basic Set!).
Martial Arts page 60 offers an optional rule on Tactics being represented by granting one's side some number of rerolls.
Ultra-Tech Drugs and Fantasy Alchemy
This isn't as direct a method of buffing as the others, but it may still be worth checking. Availability will, of course, vary wildly by setting and GM. Check B425 for the generic rule of custom design (though I haven't seen that one used often), or individual tech and/or setting books for specific samples and their stats.
The GURPS Basic Set Characters book doesn't have very many if any mechanics of the type you are referring to.
GURPS is mostly a pretty literal representational system. Things tend to happen in ways that make literal sense because of something in the game world situation. There are not many things that are only about game mechanics. And there are not many things what would realistically have the effects you mention - those are gamey conventions of other game designs that are more abstract.
How is someone in the real world going to increase their friend's strength, health, intelligence, or dexterity, or general success? They're pretty much not, except by doing specific things that help them do things (such as both shoving on the same door at once, or teaming up in other ways, or both trying something, so if one person fails, the other may succeed), not just directly making them be better.
Or if they are, it's liable to be something like magic or super powers. The GURPS Magic book has quite a few magical spells for increasing attributes and abilities of other people. There are a few such spells in the Basic Set Characters book - see Shield, Armor, Haste, Breathe Water, and No-Smell. Also see GURPS Powers for all sorts of unrealistic arbitrary super powers you could add to a character.
Other things that might have the effects you describe include drugs and potions that can alter attributes or perhaps give some advantages (often also with disadvantages - e.g. alcohol might increase your strength but decrease your dexterity and intelligence, and give you a hangover and might lead to alcoholism), or supernatural blessing effects (in the Basic Set, the GM might allow a special Enhancement to Blessed: Heroic Feats to affect others, for example, or maybe see GURPS Religion, a 3e book, for more ideas.) Another thing some other GMs and I have done is have some religious/spiritual/supernatural blessing effects where appropriate people can gain some limited "Luck" (q.v. the advantage) through blessings which can be given by holy people).