GURPS used their 3e TL scale for their adaptation of G:T and thie 4e TL scale for GT:ISW.
Approximately it converts like this
GTL9 = TTL9
GTL10 = TTL10/11
GTL11 = TTL12
Well, IIRC, TL 10 in GURPS space included Antimatter and slow FTL communications... I don't see it as TL 10-11 for Traveller.
GURPS:4e has a different TL paradigm than GURPS:3e.
In generic GURPS:4e, GTL:8 is the modern day today, and GTL12 is the HIGHEST tech-level that officially exists, and is the equivalent of Star Trek and/or the Ancients (i.e. "technological magic").
The scale between GTL8 and GTL12 is based on reasonably conservative hard-science projections based on modern understanding and speculation. Anything other than Conservative Hard Science projections are defined in 4e as "super-science technologies", and have no officially assigned GTL (although they generally provide a suggested GTL). Superscience is entirely the domain of the GM (to be assigned or not assigned to his/her universe as he/she wishes) and is denoted by a "carat" in front of the GTL number (e.g. TL^10). For Traveller, things like Jump Drive, gravitics, nuclear dampers, FTL Communications etc., would all be superscience technologies assigned to the OTU setting at tech-levels set by the game designers for their setting (or forbidden/impossible technologies), on a case-by-case basis.
In 4e, GTL12 (and GTL^12) are technologies beyond golden-age 3I understanding (i.e. Ancients-level tech, etc). GTL11/GTL^11 would be the highest GTL available in any OTU setting based on GURPS UltraTech 4e.
I would think the conversion would be approximately:
GTL 9 = TTL 9-10
GTL 10 = TTL 11-12
GTL 11 = TTL 13-16
GTL 12 = TTL 17/18+
Of course, it is not a perfect conversion going from one TL system to the other. While the ISW Era supposedly has a max TTL of 12, there are suggestion from some T4 materials (notably T4: Emperor's Arsenal) that the ISW and RoM eras did reach higher TLs in some limited fields before the Long Night.