I assume it is not there because the Updater app detects that you do not have the original storage layout anymore, but a different one.
The online developer community XDA wrote a nice tutorial, how to flash the FP1 binaries. If you use the following file to flash
1.8.7 FP1U and FP1 with upgraded partition layout:
FP1_Fairphone_OS_v1_8_7_FUSE_Image_2015-08-14.zip (~200 MB)
md5sum: 2d7a2e804b976624dfc89707dd9a546b
you should directly land on the latest version of FP OS (1.8.7 in our case) and will be offered the option to install Google Apps.
You’ll find more explanation here:
If you have further questions, we are here to help!