What is a typical value for the gain of a smartphone's Bluetooth (or Wi-Fi) antenna?
Some time ago, I myself had to look that up and the answer is pretty complex. 

One of the big issues is that in path-loss formulas, it is usually taken for granted that the antennas are pointing to each-other with their highest gain directions and therefore only highest gain is used for calculations. 

In reality, you have a 3D figure showing you the gain of the physical antenna. That figure will have a shape of its own. While, as Electrical Architect mentioned in his answer, it would be very good if you didn't have to keep track of your direction when using portable devices, in reality the gain figures can be very non-spherical and they are frequency-dependent as well.

That is why, in a datasheet for an antenna, you will be able to find several gain values such as peak maximum gain and band-edge maximum gain. Those values are not particularly interesting, because they're the best case. You, as the user, do not know which antenna is in your device or how it is positioned, even if you manage to find the datasheet.  

If you dig a bit more through the datasheets, you'll be able to find average gain (sometimes even for a frequency!), or mean effective gain or similar value. This is a more meaningful value, since it can give you a number that does not depend on the orientation as much.  

Finally, you also might also find a minimum gain value somewhere in a table on on a graph. If you want to make the absolute worst case calculation for your system, you can use this, but be warned: Sometimes the difference between the maximum and minimum value can be over 20 dB!

OK, so enough theory,you don't have the datasheets, so let me give you some values: I'd say that you can use around -1.1 dB to -2 dB as a gain figure for 2.4 GHz  to 2.5 GHz chip antennas that can be currently easily found. Keywords for Google that produce datasheets are: WiFi SMD antennas

Here are some datahseets that I read:
http://www.mt-system.ru/sites/default/files/documents/w3008.pdf
http://ww1.microchip.com/downloads/en/DeviceDoc/Acc-Antanova-Chip-Ant-DS.pdf  http://www.pulseelectronics.com/download/2987/w3108pdf/pdf