There are three ways to create vertical navigation courses that have output signals displayed to the pilot and coupled to autopilots to follow the slanted course. The oldest and most familiar is the ILS. An ILS combines two separate signals, lateral (localizer) and vertical (glideslope).
Starting in the 1970's, RNAV courses were created, initially using distance/angle from VOR's to define arbitrary points in space, but later adding other navigation aids (INS, OMEGA, Loran, DME/DME, etc) to define them. For general aviation, RNAV receivers were used for lateral navigation (the King KSN-80, for example), but airline FMS systems used them for 3-D navigation.
Vertical courses were based on a RNAV defined point and the baro-altitude of that point to create a sloped course, called a baro-VNAV course. Much later in time, when GPS was introduced, baro-VNAV slopes were created from GPS waypoints and a baro-altitude at the waypoint. Deviation signals (both lateral and vertical) are sent to a CDI to monitor progress on both the lateral and vertical components of the course, and these deviations are sent to an analog autopilot to hold course. Alternatively, digital pitch and roll commands can be sent to a digital autopilot operated in GPSS and GPSV modes to track the sloped course, avoiding the need for a CDI.
These VNAV courses are now found in some popular GPS units for general aviation; the G1000W and Chelton, for example. In these devices, in the enroute and terminal phases, baro-VNAV courses are used for step down fixes or pilot selected climbs and descents. All of these can be autopilot coupled. Each of them use a digital autopilot to track these courses. Chelton also uses baro-VNAV courses on all approaches (GPS, ILS, NDB); the only GA unit that does so (in my knowledge), with slopes defined by the particular procedure.
The third method was introduced when the Wide Area Augmentation System (WAAS) was added to the Global Navigation Satellite System (GNSS). Sloped courses here are defined by the 3-D position solution from your GPS. Each point on the course has both a lateral and vertical position to define it. To track these courses a GPS receiver that computes your position 5 times per second (rather than once per second with non-WAAS units) is required, and the altitude solution needs to be sufficiently accurate (requires WAAS error corrections) to allow the use of GPS for vertical approaches. In WAAS GPS navigators, there are 3 approaches that have a GPS vertical course; LNAV+V, LNAV/VNAV, and LPV. The altitude accuracy is a few meters.
But what kinds of signals are available to autopilots on these GPS approach courses? All GPS units send out both analog NAV/GS signals to a CDI, and digital roll signals (GPSS) that go directly to a digital autopilot or to a roll steering converter (which converts them to analog heading commands). But some also send digital pitch signals (GPSV) directly to a digital autopilot to track a vertical course. A little known fact is that the G430W/530W, the G480, and the new GTN 6xx/7xx series Touch Screen Navigators all generate these pitch signals and you can track them in GPSV mode (I do that in my Lancair ES with the TruTrak Sorcerer). But for now, it is a rare autopilot with GPSV mode. Nonetheless, there is a clear trend away from legacy analog autopilots and towards digital autopilots. TruTrak has made them for some time, but they are not certified. They are however widely used in experimental aircraft. Avidyne now has a DFC90 (GPSS for lateral only) and DFC100 (lateral and VNAV), and the G1000W has a GFC700 with both modes as part of its system.