After debian install onto guruplug, I’ve noticed few errors:

[ 10.445696] libertas_sdio mmc0:0001:1: firmware: failed to load libertas/sd8688_helper.bin (-2)
[ 10.454505] libertas_sdio mmc0:0001:1: Direct firmware load for libertas/sd8688_helper.bin failed with error -2
[ 10.605080] libertas_sdio mmc0:0001:1: firmware: failed to load sd8688_helper.bin (-2)
[ 10.613113] libertas_sdio mmc0:0001:1: Direct firmware load for sd8688_helper.bin failed with error -2
[ 10.622488] libertas_sdio: failed to find firmware (-2)
[ 10.769048] btmrvl_sdio mmc0:0001:2: firmware: failed to load mrvl/sd8688_helper.bin (-2)
[ 10.777317] btmrvl_sdio mmc0:0001:2: Direct firmware load for mrvl/sd8688_helper.bin failed with error -2
[ 10.786956] Bluetooth: Failed to download helper!
[ 10.791706] Bluetooth: Downloading firmware failed!

Also, neither Bluetooth nor Wifi were working.

To be able to install the libertas firmare You must enable non-free software in Your apt source configuration:

> cat /etc/apt/sources.list
# deb http://ftp.lt.debian.org/debian/ stretch main

deb http://ftp.us.debian.org/debian/ stretch main
deb-src http://ftp.us.debian.org/debian/ stretch main

deb http://security.debian.org/debian-security stretch/updates main
deb-src http://security.debian.org/debian-security stretch/updates main

Append “non-free” to Your main deb sources to look like this:

> cat /etc/apt/sources.list
# deb http://ftp.lt.debian.org/debian/ stretch main

deb http://ftp.us.debian.org/debian/ stretch main non-free
deb-src http://ftp.us.debian.org/debian/ stretch main non-free

deb http://security.debian.org/debian-security stretch/updates main
deb-src http://security.debian.org/debian-security stretch/updates main

Now just run the apt-get update, and Your package should be visible:

> apt-get update 
Hit:1 http://ftp.us.debian.org/debian stretch InRelease
Hit:2 http://security.debian.org/debian-security stretch/updates InRelease
Get:3 http://ftp.us.debian.org/debian stretch/non-free Sources [80.9 kB]
Get:4 http://ftp.us.debian.org/debian stretch/non-free armel Packages [52.0 kB]
Get:5 http://ftp.us.debian.org/debian stretch/non-free Translation-en [80.1 kB]
Fetched 213 kB in 2s (79.7 kB/s) 
Reading package lists... Done
> apt-cache search sd8688_helper.bin
firmware-libertas - Binary firmware for Marvell wireless cards
> apt-get install firmware-libertas
Reading package lists... Done
Building dependency tree 
Reading state information... Done
The following NEW packages will be installed:
firmware-libertas
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 2,949 kB of archives.
After this operation, 8,832 kB of additional disk space will be used.
Get:1 http://ftp.lt.debian.org/debian stretch/non-free armel firmware-libertas all 20161130-2 [2,949 kB]
Fetched 2,949 kB in 1s (2,836 kB/s) 
Selecting previously unselected package firmware-libertas.
(Reading database ... 23795 files and directories currently installed.)
Preparing to unpack .../firmware-libertas_20161130-2_all.deb ...
Unpacking firmware-libertas (20161130-2) ...
Setting up firmware-libertas (20161130-2) ...

And error should be gone.