Third Imperium - A Note About Races in GURPS and Races in GURPS: Traveller
Races in GURPS
Races in the GURPS game system are essentially fixed-cost point packages paid for during character creation. For the most part, they are an array of advantages and disadvantages that reflect the characteristics of that race. One generally cannot pick and choose which racial characteristics one wishes to have out of a racial package - one is either a member of the race, with all of the benefits and drawbacks that entails, or one is not. On a side note, there are some advantages and disadvantages that are usually only available as part of a racial package, although magic and extremely high-technology could conceivably change that.
Note that racial disadvantages do not count against a character's 40 point disadvantage limit for the purpose of character creation. In a similar manner, racial quirks do not count against the character's 5 point quirk limit. It should also be noted that some racial packages have very high point totals, and as a result may be difficult or even impossible to take when using a standard 100 point character build.
Races in GURPS: Traveller
As a general rule of thumb, all of the races found in the various GURPS: Traveller sourcebooks, including GURPS: Humaniti and all of the GURPS: Alien Races books, are available as character options in the Third Imperium. I will note that all of those sourcebooks are GURPS Third Edition books, and thus the point costs for those racial packages may be inaccurate for GURPS Fourth Edition. I am in the process of converting all the the racial templates to the more recent edition, but this is a time-consuming process and likely won't be completed for quite a while. Any player wishing to play a particular race that has not yet been converted should contact me and I will do the necessary conversion.
I will also point out that the area covered by the Third Imperium and its neighbors is huge. Space is vast, and between the various interstellar civilizations detailed in the setting, there are tens of thousands of inhabited star systems. Consequently, this means that there are likely a myriad of minor alien races that are not detailed in the official GURPS: Traveller sourcebooks. With some modification, races from GURPS: Aliens, GURPS: Uplift, or GURPS: Lensman could be fit into the GURPS: Traveller setting. It is plausible that a race from GURPS: Fantasy Folk or even GURPS: Myth could be modified to fit the setting as well, although that determination would have to be made on a case-by-case basis. If a player wishes to play a particular race from one of those sources, they should contact me and we can work together to place them into the setting and establish an appropriate point cost.