# [Python 2], 74 bytes

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

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

[Try it online!][TIO-k1m5pike]

[Python 2]: https://docs.python.org/2/
[TIO-k1m5pike]: https://tio.run/##K6gsycjPM/qfnJ@SqmCroK6u/j/TNjOvoLREQ9M631ZJiSstv0ihQiEzTynb1z1EQckq31YjU6dCOzNa19gqVjM6JzVPI1PTzjhWO98aqDPaStc4lqugKDOvRCH/P9A4rtSK1GQNkPGaWoam/5UMjZS4gIQxhDSBUqYw2gzOMEewLJCYlshsAxQOGg@di8HHFDBQAgA "Python 2 – Try It Online"

Previous version for 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"

 - <s>Reverse the input</s>
 - Step <s>through</s> backwards in threes <s>appending</s> prepending the next letter
 - <s>Reverse and print</s>

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