Sorry outside of my field

I can answer your question fully when the details are set out.

Gas engine can suck and if this suction is very demanding, one can introduce a blower (~ 100 to a few hundred mm water gauge pressure above ambient) and solve the problem. A gas turbine requires a much higher injection pressure and this needs a separate compressor (see https://www.wbdg.org/resources/microturbines) unless it also functions by being drawn with the air. One manufacturer who was using this approach in early development is capstone mivcroturbine. The second approach has potential risk of back flame propagation and inevitable problems. To answer your question fully, you will need to putdown a schematic of the gas path from gasifier to the reciprocating gas engine as well as gas turbine.

This sounds like you are running a carburetor (downdraft gasifier) on a standard piston engine and are having difficulty getting the engine to run properly when the turbocharger (gas turbine) starts to develop boost. I interpret 'insufficient carbon conversion' to mean incomplete or no combustion.

If this is the case, the first thing I suspect is that the fuel bowl of the carburetor is vented to the atmosphere while the downdraft venturi is referenced to the discharge pressure of the turbine. Both the inlet to the carburetor and fuel bowl vent on the carburetor MUST be referenced to the same (inlet duct) pressure. A small line that runs from the inlet path to the vent of the fuel bowl is usually all that is required. It may be necessary to reference the fuel pump to turbocharger discharge pressure as well. This is dependent on the type of fuel pump used.

Insure the throttle shafts on the carburetor are sealed so that pressurized fuel and air will not be forced out of the carburetor into the engine compartment. Leaks of fuel and air mixed this way can become explosive with disastrous results. Viton O-rings installed on the throttle shafts are usually sufficient to prevent this from occurring.