TL;DR: According to ASHRAE 62.1, this input for carbon dioxide generation rate should be the same regardless of space type. That's why it is the same for all space types in OpenStudio library files. What you need to change between space types are the other input fields for occupants (activity schedule, occupancy schedule, and occupant density) so that EnergyPlus can properly calculate the volume of carbon dioxide generated.
Explanation
The SI unit for that carbon dioxide generation rate input input field is actually
$$\frac{L}{s*W}$$
so it's a volumetric rate of $CO_2$ divided by the activity rate of the occupants. That's why the IP unit translates to
$\frac{ft^3}{min} / \frac{Btu}{hr}$ = $\frac{ft^3*hr}{min*Btu}$
The ASHRAE 62.1-2013 figure that you mentioned is shown below. For the "maybe 1 liter per minute" of $CO_2$ generation that you referenced, I'm guessing that you are looking at the metabolic rate of ~4 met.
That, however, is for one specific metabolic rate value. Note that the $CO_2$ generation curve is a straight line, so you can easily calculate volumetric $CO_2$ generation based on metabolic rate. This generation rate input that you're asking about is the slope of that curve. The default value is 3.82E-8 $\frac{m^3}{s*W}$ or 3.82E-5 $\frac{L}{s*W}$, which you can see in OpenStudio if you create a new People load definition. This default was obtained from ASHRAE Standard 62.1-2007 value at 0.0084 $\frac{cfm}{met*person}$ over the general adult population. Because it's a straight line, this generation rate input is the same no matter what space type you are simulating.
EnergyPlus uses the activity schedule, occupancy schedule, and occupant density input fields in order to calculate the physical activity of occupants (horizontal axis of the ASHRAE 62.1 figure). If you set these three inputs to reflect gym occupants, then you should get ~4 met. These inputs are then used with the generation rate input (slope of the line in the ASHRAE 62.1 figure) in order to calculate the volumetric $CO_2$ generation (vertical axis of the ASHRAE 62.1 figure). The schedules are assigned to space types in OpenStudio, and the occupant density is assigned to the people load definition (see below).
UPDATE
The units that OpenStudio expects for the activity schedule are $\frac{W}{person}$. There is a table of metabolic rates for various activities in the EnergyPlus documentation for the People object. This has a range of 315 - 423 $\frac{W}{person}$ for "calisthenics/exercise", which is equivalent to 3 - 4 $met$ assuming each occupant has 1.8 $m^2$ of surface area. This table of metabolic rates can be found in ASHRAE Standard 55 (Table 5.2.1.2 in 55-2010).
