`gm start` fails due to missing `minimum-gas-prices`

## Summary of issue

Gaiad Manager fails to spawn both chains and nodes when using `gm start`. The issue is caused by a missing value for `minimum-gas-prices` in `ibc-*/config/app.toml` and `node-*/config/app/toml`.

## Version

`gaiad` v15.2.0
`sconfig` v0.2.0
`stoml` v0.7.1
`gm` v0.1.6

## Steps to Reproduce
1. Add configuration to gm.toml:
```[global]
add_to_hermes = false
auto_maintain_config = true
extra_wallets = 2
gaiad_binary = "~/go/bin/gaiad"
hdpath = ""
home_dir = "$HOME/.gm"
ports_start_at = 27000
validator_mnemonic = ""
wallet_mnemonic = ""

[global.hermes]
binary = "/opt/hermes/target/release/hermes" #change this path according to your setup
config = "$HOME/.hermes/config.toml"
log_level = "info"
telemetry_enabled = true
telemetry_host = "127.0.0.1"
telemetry_port = 3001

[ibc-0]
ports_start_at = 27010

[ibc-1]
ports_start_at = 27020

[node-0]
add_to_hermes = true
network = "ibc-0"
ports_start_at = 27030

[node-1]
add_to_hermes = true
network = "ibc-1"
ports_start_at = 27040
```
2. Start chains with `gm start`

- Output:

``` ibc-0 started, PID: 29856, LOG: ~/.gm/ibc-0/log
WARNING: ibc-0 failed. Check the logs with `gm log ibc-0`.
ibc-1 started, PID: 30038, LOG: ~/.gm/ibc-1/log
WARNING: ibc-1 failed. Check the logs with `gm log ibc-1`.
node-0 started, PID: 30313, LOG: ~/.gm/node-0/log
WARNING: node-0 failed. Check the logs with `gm log node-0`.
node-1 started, PID: 30591, LOG: ~/.gm/node-1/log
WARNING: node-1 failed. Check the logs with `gm log node-1`
```
- Output from `gm log ibc-1`:
```
Error: set min gas price in app.toml or flag or env variable: error in app.toml
Usage:
gaiad start [flags]
(...)
```

## Acceptance Criteria

`gm` sets a default value for `minimum-gas-prices` in `app.toml` when generating chain/node configuration.
____

## For Admin Use

- [ ] Not duplicate issue
- [ ] Appropriate labels applied
- [ ] Appropriate milestone (priority) applied
- [ ] Appropriate contributors tagged
- [ ] Contributor assigned/self-assigned

Reproduce the failure with `gm start`, then inspect the generated `ibc-*/config/app.toml` and `node-*/config/app.toml` files and use `gm log ibc-1` or `gm log node-0` to confirm the error. The work is done when generated configurations include a default `minimum-gas-prices` value and both chains and nodes start successfully.

Written by the indexing model from the issue text.