Solar power is for charging batteries that device runs off. If you want standalone GPS unit that's not connected to bike's battery, it too would need its own battery for nighttime operation, which is redundant and incurs environmental resources to build and disposed of yet another battery. Better off just to have one main battery, existing bike's battery. Then attach GPS unit to that as well as solar-panel for charging bike's battery.
There are 3 main components to GPS tracker: GPS-receiver, cellular-transmitter, central-server. The central-server is what you'd access from your phone/PC to find position of tracker. This requires service-fee to use their server.
Here's a unit with good reviews on amazon:
https://www.amazon.com/dp/B00JZIES4M
And solar battery-charger:
https://www.amazon.com/dp/B004TIPKUE