The main consideration for this is the “preset” A.K.A. video output size. When 1920X1080 output is selected, this is similar to a 4x scale. However with the gbscontrol firmware, the final result is not necessarily an integer multiple of the source. It very much depends on the selected "preset" and additional options. Generally, vertical scaling uses an integer scale factor, while horizontal scaling uses an oversampling method which can cause some variation but each “preset” maximizes the sampling and processing capability of the Trueview 5725 chip which is responsible for scaling. The Trueview 5725 over samples the signal then reduces it, and has the VDS format the signal to the correct output timings while filtering to get rid of chroma subsampling issues that may occur.
This approach achieves an artifact free, slightly interpolated but still well defined output, that does not shimmer when scrolling and works with all common source pixel clocks to always deliver an overall good picture.
It's also worth noting that during the reduction stage that each scan line is written to SDRAM. The VDS (output timing generator) then reads each scan line from another position in the SDRAM that belongs to the same field. This short buffering in SDRAM produces the progressive / interlace fast resolution switching that prevents video from de-syncing during 240P to 480i resolution changes.
This entire process can be thought of like auto-pilot for video profiling and is why the Gbscontrol firmware does not need profiles like the OSSC. There is no need to specifically tweak video setting to get the best picture from the Gbscontrol firmware.
This auto-pilot does come at the cost of some image clarity but still produces a very good image with virtually no input from the user. Plug in your video source, set the video resolution preset and play!
I owe much of this explanation to Rama (the creator of the Gbscontrol firmware) but I think it does a good job of explaining how the Gbscontrol firmware and GBS hardware scale video.