Yes — an antenna only performs on the band it was cut for, and transmitting through the wrong one wrecks range and can damage the radio. GMRS lives in UHF around 462-467 MHz, ham spans its own bands, and CB sits way down near 27 MHz, which is why CB whips run so long. Buy the antenna for the service you actually use; no honest antenna does everything.

Center of the roof, if you can live with it — most mobile antennas use the metal beneath them as a ground plane, and roof-center throws the most even pattern in every direction. Hood-lip, fender, and rack mounts all work with some directional compromise and easier garage clearance. Keep the antenna clear of racks and tents, which shadow the signal on whichever side they sit.

No — gain does not add power, it flattens the signal toward the horizon. Across open desert that means free range; in steep terrain the flattened pattern shoots over the ridge instead of down into the next drainage, and a lower-gain antenna with a rounder pattern wins. Match gain to the country you actually drive, not to the biggest number on the packaging.

If branches will touch it, yes — a spring lets the whip fold and recover instead of bending or snapping at the mount. Rigs running tight tree cover benefit most; open-desert trucks can usually skip it and gain a steadier whip at highway speed. Either way, buy a solid mount, because the mount absorbs the leverage whether or not a spring is in the stack.

Terrain, almost always — GMRS and ham VHF/UHF are largely line-of-sight, so a ridge between rigs beats any antenna upgrade. After terrain, check the fundamentals: a proper ground plane, undamaged coax with clean connections, and SWR tuned at the antenna's actual mounting location, since every install behaves differently. Range numbers quoted across open ground mean very little in a canyon.