Electronic – the antenna gain of a mobile phone

Typically the 'host' refers to the master device/entity that belongs to the side of the computer. It could be inside computer or external to a computer. [it could be a microcontroller , but the point is host is more closer to you when you writing software to work with].

The main idea is before you communicate with the other device, you communicate with the host, for a example hard disk controller is a host, USB controller chip is a host.

So according to your GSM module thing, The chip hardware/software that parsing the AT command set and act as a front end is refereed as a host.

Your Manual/Specification probably should have a chapter called terms and Abbreviations. Turn out for that page and you'll find the specific abbreviation which is specific to your module.

There are no antennas that are truly omnidirectional without a lot of complication especially for a cell-phone. However, if you think about it, you do want to be able to hold your handset and turn in any direction to face north, south, east or west and expect it to work. A directional loop antenna defeats the objective of being able to face any direction and still work.

You could argue that laying flat on the ground (whilst on the phone) is "not a requirement" and therefore you shouldn't expect this to work in all directions. However, the reception and transmission of radio waves is quite complex to understand (and a bit unpredictable) and these can make the "normal" expectations better or worse. Here's what a dipole's field looks like: -

You can see that it extends in all directions horizontally the same but in the vertical direction there is no field (receiving or transmitting).

The design of the "system" also provides different frequencies for any particular handset to operate on and sometimes when reception might be poor at one frequency, moving to an adjacent frequency might make a significant improvement. Ditto for transmission.