# [Python 2], 84 bytes

<!-- language-all: lang-python -->

    i=input()[::-1];o=""
    for x in"kMGT ":o+=(i,i[0:3]+x)[len(i)>3];i=i[3:]
    print o[::-1]

[Try it online!][TIO-k1m542z5]

[Python 2]: https://docs.python.org/2/
[TIO-k1m542z5]: https://tio.run/##ZY3LDoIwEEX3/YpmNrSiCVDxUVK3rty5a7rCGiealhBM8OsrBh8Im3vPmWRmqkdz8S4LpT9ZqmgURQEVuureMK6lXKSm8AqAnH1NW4oOrof9kYL0sWI4R51IYeKW65t1DPlOmKJb10IaUtXoGur7I6E7TGxrS/Z6xGdpHiDNgHQh@ly@K//06gvrH20GuB1y8icjG@vEp4MEng "Python 2 – Try It Online"

 - Reverse the input
 - Step through in threes appending the next letter
 - Reverse and print

Not sure that this is the most efficient approach but it's the best I can come up with at the moment.