Comment 1 for bug 1729738

On 16.04 I have zoom working in all described situations; touchpad (two fingers), keyboard (ctrl+(+/-)). UI buttons and scrollwheel on mouse also works for me.

For the keyboard shortcut, are you maybe having another application bound to ctrl++?

On my 16.04 system I've got these versions installed of package and main dependency for the map (libosmgpsmap) (https://github.com/nzjrs/osm-gps-map):

$ dpkg -l|grep gpxviewer
ii gpxviewer 0.5.2-1 all views GPS traces collected in the GPX format

$ dpkg -l|grep osmgpsmap
ii gir1.2-osmgpsmap-1.0 1.1.0-1 amd64 GTK+ library to embed OpenStreetMap maps - Python bindings
ii libosmgpsmap-1.0-1:amd64 1.1.0-1 amd64 GTK+ library to embed OpenStreetMap maps

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 16.04.4 LTS
Release: 16.04
Codename: xenial

The repository seems to carry the exact same versions for xenial (16.04) and artful (17.10):

https://packages.ubuntu.com/search?keywords=gpxviewer&searchon=names&suite=all&section=all
https://packages.ubuntu.com/search?keywords=libosmgpsmap&searchon=names&suite=all&section=all